define _heim_base_mutex if needed

base/heimbase.c

@@ -64,6 +64,10 @@ struct heim_base_mem {
 #define PTR2BASE(ptr) (((struct heim_base *)ptr) - 1)
 #define BASE2PTR(ptr) ((void *)(((struct heim_base *)ptr) + 1))
 
+#ifdef HEIM_BASE_NEED_ATOMIC_MUTEX
+HEIMDAL_MUTEX _heim_base_mutex = HEIMDAL_MUTEX_INITIALIZER;
+#endif
+
 /*
  * Auto release structure
  */