systemctl: remove compiler warning (#6717)

systemctl: remove compiler warning (#6717)

913c1916 changed _ACTION_INVALID to negative, changing the enum to a
signed type.  Take care to avoid comparing it with an unsigned type.

../src/systemctl/systemctl.c: In function ‘start_unit’:
../src/systemctl/systemctl.c:3107:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                 assert(arg_action < ELEMENTSOF(action_table));