common: hoist hash_str helper into its own header.

diff --git a/common/Makefile b/common/Makefile
index 69befa16..d6060642 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -121,6 +121,7 @@ COMMON_HEADERS_NOGEN := $(COMMON_SRC_NOGEN:.c=.h)	\
 	common/ecdh.h					\
 	common/errcode.h				\
 	common/gossip_constants.h			\
+	common/hash_str.h				\
 	common/hsm_version.h				\
 	common/htlc.h					\
 	common/jsonrpc_errors.h				\
diff --git a/common/hash_str.h b/common/hash_str.h
new file mode 100644
index 00000000..111073e9
--- /dev/null
+++ b/common/hash_str.h
@@ -0,0 +1,11 @@
+#ifndef LIGHTNING_COMMON_HASH_STR_H
+#define LIGHTNING_COMMON_HASH_STR_H
+#include "config.h"
+#include <common/pseudorand.h>
+
+static inline size_t hash_str(const char *str)
+{
+	return siphash24(siphash_seed(), str, strlen(str));
+}
+
+#endif /* LIGHTNING_COMMON_HASH_STR_H */
diff --git a/common/utils.c b/common/utils.c
index 860cd446..5dcbee5a 100644
--- a/common/utils.c
+++ b/common/utils.c
@@ -253,4 +253,3 @@ void *membuf_tal_resize(struct membuf *mb, void *rawelems, size_t newsize)
 	tal_resize(&p, newsize);
 	return p;
 }
-
diff --git a/lightningd/plugin.h b/lightningd/plugin.h
index b7f03360..5bc0c3af 100644
--- a/lightningd/plugin.h
+++ b/lightningd/plugin.h
@@ -2,6 +2,7 @@
 #define LIGHTNING_LIGHTNINGD_PLUGIN_H
 #include "config.h"
 #include <ccan/intmap/intmap.h>
+#include <common/hash_str.h>
 #include <lightningd/jsonrpc.h>
 #include <lightningd/lightningd.h>
 
@@ -24,11 +25,6 @@ struct plugin_subscription {
 	const char *topic;
 };
 
-static inline size_t hash_str(const char *str)
-{
-	return siphash24(siphash_seed(), str, strlen(str));
-}
-
 static inline const char *plugin_subscription_key(const struct plugin_subscription *ps)
 {
 	return ps->topic;
diff --git a/plugins/bkpr/account.c b/plugins/bkpr/account.c
index c8b07468..c01bafef 100644
--- a/plugins/bkpr/account.c
+++ b/plugins/bkpr/account.c
@@ -4,6 +4,7 @@
 #include <ccan/json_out/json_out.h>
 #include <ccan/str/str.h>
 #include <ccan/tal/str/str.h>
+#include <common/hash_str.h>
 #include <common/memleak.h>
 #include <common/mkdatastorekey.h>
 #include <common/node_id.h>
@@ -14,11 +15,6 @@
 #include <plugins/libplugin.h>
 #include <wire/wire.h>
 
-static size_t hash_str(const char *str)
-{
-	return siphash24(siphash_seed(), str, strlen(str));
-}
-
 static const char *account_key(const struct account *account)
 {
 	return account->name;
diff --git a/plugins/bkpr/onchain_fee.c b/plugins/bkpr/onchain_fee.c
index 79b6f15a..8a6cc14b 100644
--- a/plugins/bkpr/onchain_fee.c
+++ b/plugins/bkpr/onchain_fee.c
@@ -6,6 +6,7 @@
 #include <ccan/intmap/intmap.h>
 #include <ccan/json_out/json_out.h>
 #include <ccan/tal/str/str.h>
+#include <common/hash_str.h>
 #include <common/json_stream.h>
 #include <common/memleak.h>
 #include <common/mkdatastorekey.h>
@@ -57,11 +58,6 @@ struct ordered_ofees {
 	struct onchain_fee **ofs;
 };
 
-static size_t hash_acctname(const char *str)
-{
-	return siphash24(siphash_seed(), str, strlen(str));
-}
-
 static const char *onchain_fees_keyof(const struct ordered_ofees *ofees)
 {
 	return ofees->ofs[0]->acct_name;
@@ -75,7 +71,7 @@ static bool onchain_account_eq(const struct ordered_ofees *ofees,
 
 HTABLE_DEFINE_NODUPS_TYPE(struct ordered_ofees,
 			  onchain_fees_keyof,
-			  hash_acctname,
+			  hash_str,
 			  onchain_account_eq,
 			  ofees_hash);