Fix building of RPMs and get RPM builds working with a prefix other than /usr again.

(Change I567269d8e45e6543d9e39dbedc49830adf7edb9f made the desktop files
install outside of /usr/share/ .)

RPMs now build and work regardless of the prefix but the desktop integration stuff
only works if the prefix is /usr or (if you're lucky) /usr/local .

Change-Id: If20e0127a044eac1ba099f959a90d068c4bb2ae5
Reviewed-on: https://code.wireshark.org/review/5600
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>