Explicitly define low_hashmem to be extern.

Otherwise there is one instance per .so-file, which was not intended.

src/pike_memory.h

@@ -159,7 +159,7 @@ PMOD_EXPORT void reverse(char *memory, size_t nitems, size_t size);
 PMOD_EXPORT void reorder(char *memory, INT32 nitems, INT32 size,INT32 *order);
 
 #if (defined(__i386__) || defined(__amd64__)) && defined(__GNUC__)
-PMOD_EXPORT
+extern PMOD_EXPORT
 #ifdef __i386__
 __attribute__((fastcall)) 
 #endif