Add extern C to upb_alloc_global

upb/upb.h

@@ -159,8 +159,16 @@ UPB_INLINE void upb_free(upb_alloc *alloc, void *ptr) {
 
 /* The global allocator used by upb.  Uses the standard malloc()/free(). */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern upb_alloc upb_alloc_global;
 
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
 /* Functions that hard-code the global malloc.
  *
  * We still get benefit because we can put custom logic into our global