debian/fixcontrol.master: fix libsystemd-dev conditional logic
debian/fixcontrol.master: fix libsystemd-dev conditional logic

Change the guard from ENABLE_SYSTEMD to PMDA_SYSTEMD.
Verbose comment also added to explain why, as this is not immediately
obvious.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp