Follow up to r247960 and rr247960 by also amending ctfmerge. For the
only other case where STT_FILE symbols are used, in symit_next() in
cddl/contrib/opensolaris/tools/ctf/cvt/input.c, save the basename of the
symbol, instead of the full pathname.

Reported by: avg
Tested by: avg, jimharris
MFC after: 1 week