clang: suppress warnings

Use uint32_t for value size.

    lib/str.c:2528:90: warning: passing 'int *' to
          parameter of type 'uint32_t *' (aka 'unsigned int *') converts between
          pointers to integer types with different sign [-Wpointer-sign]
      ...*ri = (grn_rset_recinfo *)grn_obj_get_value_(ctx, a->obj, id, &vs);
                                                                       ^~~
    lib/db.h:232:89: note: passing argument to
          parameter 'size' here
      ...*ctx, grn_obj *obj, grn_id id, uint32_t *size);
                                                  ^
    lib/str.c:2535:90: warning: passing 'int *' to
          parameter of type 'uint32_t *' (aka 'unsigned int *') converts between
          pointers to integer types with different sign [-Wpointer-sign]
      ...*ri = (grn_rset_recinfo *)grn_obj_get_value_(ctx, a->obj, id, &vs);
                                                                       ^~~
    lib/db.h:232:89: note: passing argument to
          parameter 'size' here
      ...*ctx, grn_obj *obj, grn_id id, uint32_t *size);
                                                  ^

lib/str.c

@@ -2502,7 +2502,7 @@ grn_substring(grn_ctx *ctx, char **str, char **str_end, int start, int end, grn_
 static void
 grn_text_atoj(grn_ctx *ctx, grn_obj *bulk, grn_obj *obj, grn_id id)
 {
-  int vs;
+  uint32_t vs;
   grn_obj buf;
   if (obj->header.type == GRN_ACCESSOR) {
     grn_accessor *a = (grn_accessor *)obj;