FIX: setuid Flag Was Removed in Debian Packages

The CMakeLists.txt installs `cvmfs_suid_helper` *with* the setuid flag correctly set. Unfortunately, during package building the **debhelper** [dh_fixperms](http://man.he.net/man1/dh_fixperms) removes the flag automagically.

**Note:** This fix is untested, I'd like to try it using the nightly build procedure.