Fix: libcrmservice: Enclose dup_file_path() in SUPPORT_NAGIOS guard

This function is only called inside a SUPPORT_NAGIOS guard. So if
Pacemaker is compiled with Nagios disabled, the build fails due to a
"defined but not used" error.

If we call the function somewhere else later, we can unguard it.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>