clang: suppress a warning

    lib/db.c:3098:7: warning: initializing 'int *'
          with an expression of type 'uint32_t *' (aka 'unsigned int *') converts
          between pointers to integer types with different sign [-Wpointer-sign]
          GRN_TABLE_EACH(ctx, table2, 0, 0, id, &key, &key_size, &value2, {
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It seems that we should treat all key sizes as unsigned int.