clang: suppress warnings

Use uint32_t for object type. It should be uint8_t but GRN_B_DEC()
requires uint32_t for result space. So I use uint32_t instead of
uint8_t.

    lib/expr.c:440:18: warning: comparison of
          constant 32 with expression of type 'grn_expr_pack_type' is always false
          [-Wtautological-constant-out-of-range-compare]
        if (GRN_TYPE <= type && type <= GRN_COLUMN_INDEX) { /* error */ }
            ~~~~~~~~ ^  ~~~~
    lib/expr.c:440:34: warning: comparison of
          constant 72 with expression of type 'grn_expr_pack_type' is always true
          [-Wtautological-constant-out-of-range-compare]
        if (GRN_TYPE <= type && type <= GRN_COLUMN_INDEX) { /* error */ }
                                ~~~~ ^  ~~~~~~~~~~~~~~~~
    lib/expr.c:467:20: warning: comparison of
          constant 32 with expression of type 'grn_expr_pack_type' is always false
          [-Wtautological-constant-out-of-range-compare]
          if (GRN_TYPE <= type && type <= GRN_COLUMN_INDEX) {
              ~~~~~~~~ ^  ~~~~
    lib/expr.c:467:36: warning: comparison of
          constant 72 with expression of type 'grn_expr_pack_type' is always true
          [-Wtautological-constant-out-of-range-compare]
          if (GRN_TYPE <= type && type <= GRN_COLUMN_INDEX) {
                                  ~~~~ ^  ~~~~~~~~~~~~~~~~