systemctl: avoid warning about signed-unsigned compare on 32 bits

../src/systemctl/systemctl-list-units.c: In function ‘output_units_list’:
../src/systemctl/systemctl-list-units.c:112:75: warning: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Wsign-compare]
  112 |         for (const UnitInfo *u = unit_infos; unit_infos && u - unit_infos < c; u++) {
      |                                                                           ^

The pointer difference is signed. I don't know why gcc warns on 32bits, but
not otherwise.

gcc-10.2.1-9.fc33.x86_64, -m32