Get rid of rpath when we're building RPMs.

Fedora prohibits it, we don't need it, and it gets in the way some times.

Change-Id: I92cee959f471866ff90efe9f96e4fa519c357dd9
Reviewed-on: https://code.wireshark.org/review/2763
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>