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);
                                                  ^