[Fix] Enabled jemalloc thread safety.

Credit to @DavidADoran for reporting the problem https://github.com/MSOpenTech/redis/issues/376 and providing
the code to reproduce the bug.

deps/jemalloc-win/include/jemalloc/internal/mutex.h

@@ -45,7 +45,7 @@ struct malloc_mutex_s {
 extern bool isthreaded;
 #else
 #  undef isthreaded /* Undo private_namespace.h definition. */
-#  define isthreaded false
+#  define isthreaded true
 #endif
 
 bool    malloc_mutex_init(malloc_mutex_t *mutex);