contrib: make spec file to work with any version of RHEL

The macro _initddir vs _initrddir is or isn't available depending on
version, so force the _initddir to be alias if needed.

Reference: https://fedoraproject.org/wiki/Packaging:RPMMacros?rd=Packaging/RPMMacros#Macros_mimicking_autoconf_variables
Signed-off-by: Sami Kerola <kerolasa@iki.fi>