girrepository: Replace usage of gsize with standard size_t

We just do a safe s/gsize/size_t/ replacement here without doing any
changes to places in which different size of size_t and gsize may be
actually different and create troubles.

girepository/girwriter.c

@@ -1249,7 +1249,7 @@ write_union_info (const char *ns,
   const char *type_init;
   const char *func;
   gboolean deprecated;
-  gsize size;
+  size_t size;
 
   name = gi_base_info_get_name ((GIBaseInfo *)info);
   deprecated = gi_base_info_is_deprecated ((GIBaseInfo *)info);