lib:ldb:tests: Fix signedness build error

lib:ldb:tests: Fix signedness build error

lib/ldb/tests/ldb_filter_attrs_in_place_test.c:836:55: error: pointer
targets in passing argument 1 of ‘_assert_string_equal’ differ in
signedness [-Werror=pointer-sign]
  836 |         assert_string_equal(msg->elements[0].values[0].data,
      |                                                       ^
      |                                                       |
      |                                                       uint8_t * {aka unsigned char *}

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>