packaging: add find and which dependencies

packaging: add find and which dependencies

The postrotate script in src/logrotate.conf uses the which and find utilities
to do its work. Although logrotate itself is only a Recommends, I think which
and find are so ubiquitous that it makes sense to have them as hard
dependencies.

Also, I checked and the package names which and findutils are the same on all
the RPM distros we are currently targeting in the spec file.

In Debian, find is also in a package called findutils while the package
containing 'which' is called debianutils.

Signed-off-by: Nathan Cutler <ncutler@suse.com>