test: compile test-utmp.c only if UTMP is enabled
When compiling with -D utmp=false the compilation fails with:
../../git/systemd/src/test/test-utmp.c: In function ‘test_dump_run_utmp’:
../../git/systemd/src/test/test-utmp.c:21:9: error: cleanup argument not a function
21 | _unused_ _cleanup_(utxent_cleanup) bool utmpx = false;
| ^~~~~~~~
../../git/systemd/src/test/test-utmp.c:23:17: error: implicit declaration of function ‘utxent_start’ [-Werror=implicit-function-declaration]
23 | utmpx = utxent_start();
| ^~~~~~~~~~~~
any many other errors
Add a conditional to compile test-utmp.c only if ENABLE_UTMP is true.
(cherry picked from commit 41cac2a8b98fc5faebe942c697b17e109822342d)