test: compile test-utmp.c only if UTMP is enabled

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.