mkosi: install test dependencies for EnterNamespace= test (#39268)

The test for the EnterNamespace= feature [0] has been both broken and
disabled since the migration to the mkosi framework, as it's missing the
libdw.pc file for pkg-config, so the test is skipped completely, and
it's also missing gcc to actually build the test binary.

[0] Part of TEST-87-AUX-UTILS-VM.coredump.sh