MFC r248991:

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