ceph: use col_add_str instead of col_set_str

The argument is no longer guaranteed to be a const that hangs around.

Bug: 10395
Change-Id: I3c658f6db5927e09a9a38f9ec0fbce153b3e1bc2
Reviewed-on: https://code.wireshark.org/review/3709
Reviewed-by: Michael Mann <mmann78@netscape.net>