Add a memory barrier to the mutex initialization logic, try to work around
an issue reported by WebKit.

FossilOrigin-Name: 11a9a786ec06403addb47f5c6fb142b382fae522
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 89b3d79..eeb0b99 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -3191,6 +3191,7 @@
   sqlite3_mutex *sqlite3MutexAlloc(int);
   int sqlite3MutexInit(void);
   int sqlite3MutexEnd(void);
+  void sqlite3MemoryBarrier(void);
 #endif
 
 sqlite3_int64 sqlite3StatusValue(int);