test: reintroduce m4 dependency for TEST-06-SELINUX

m4 is required to build the test SELinux module:

```
[   31.321789] sh[483]: /bin/sh: line 1: m4: command not found
[   31.882668] sh[488]: Compiling targeted systemd_test module
[   32.120862] sh[492]: /bin/sh: line 1: m4: command not found
[   32.159897] sh[458]: make: *** [/usr/share/selinux/devel/include/Makefile:156: tmp/systemd_test.mod] Error 127
```