Add missing files to COMMON_SFILES

Add missing files to COMMON_SFILES

While working on the previous patch, I found a few .o files whose
corresponding .c file was not mentioned in Makefile.in.  This patch
fixes the problem.  I pulled this out separately to make it simpler to
review.

ChangeLog
2017-11-27  Tom Tromey  <tom@tromey.com>

	* Makefile.in (COMMON_OBS): Remove filename-seen-cache.o,
	registry.o, thread-fsm.o, debug.o.
	(COMMON_SFILES): Add filename-seen-cache.c, registry.c,
	thread-fsm.c, debug.c.