cmake: add empty RPATH to ceph-diff-sorted

cmake: add empty RPATH to ceph-diff-sorted

This fixes a transient FTBFS on openSUSE:

[ 5365s] -- Installing: /home/abuild/rpmbuild/BUILDROOT/ceph-15.2.4.337+g55cec95eaf-1.1.x86_64/usr/bin/ceph-diff-sorted
[ 5365s] CMake Error at src/tools/cmake_install.cmake:230 (file):
[ 5365s]   file RPATH_CHANGE could not write new RPATH:
[ 5365s]
[ 5365s]     /usr/lib64/ceph
[ 5365s]
[ 5365s]   to the file:
[ 5365s]
[ 5365s]     /home/abuild/rpmbuild/BUILDROOT/ceph-15.2.4.337+g55cec95eaf-1.1.x86_64/usr/bin/ceph-diff-sorted
[ 5365s]
[ 5365s]   No valid ELF RPATH or RUNPATH entry exists in the file;

Fixes: https://tracker.ceph.com/issues/46553
Signed-off-by: Nathan Cutler <ncutler@suse.com>