clang: suppress a warning

    lib/expr.c:437:37: warning: passing
          'const uint8_t *' (aka 'const unsigned char *') to parameter of type
          'const char *' converts between pointers to integer types with different
          sign [-Wpointer-sign]
        v = grn_expr_add_var(ctx, expr, ns ? p : NULL, ns);
                                        ^~~~~~~~~~~~~