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.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+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.
+
 2017-11-27  Tom Tromey  <tom@tromey.com>
 
 	* Makefile.in (COMMON_SFILES): New.

gdb/Makefile.in

@@ -957,6 +957,7 @@ COMMON_SFILES = \
 	d-namespace.c \
 	d-valprint.c \
 	dbxread.c \
+	debug.c \
 	demangle.c \
 	dictionary.c \
 	disasm.c \
@@ -976,6 +977,7 @@ COMMON_SFILES = \
 	f-lang.c \
 	f-typeprint.c \
 	f-valprint.c \
+	filename-seen-cache.c \
 	filesystem.c \
 	findcmd.c \
 	findvar.c \
@@ -1048,6 +1050,7 @@ COMMON_SFILES = \
 	record-full.c \
 	regcache.c \
 	reggroups.c \
+	registry.c \
 	reverse.c \
 	rust-lang.c \
 	selftest-arch.c \
@@ -1070,6 +1073,7 @@ COMMON_SFILES = \
 	target-descriptions.c \
 	target-memory.c \
 	thread.c \
+	thread-fsm.c \
 	tid-parse.c \
 	top.c \
 	trad-frame.c \
@@ -1544,12 +1548,10 @@ COMMON_OBS = $(DEPFILES) $(CONFIG_OBS) $(YYOBJ) \
 	job-control.o \
 	common-regcache.o \
 	common-utils.o \
-	debug.o \
 	environ.o \
 	errors.o \
 	exec.o \
 	fileio.o \
-	filename-seen-cache.o \
 	filestuff.o \
 	format.o \
 	gdb_tilde_expand.o \
@@ -1558,13 +1560,11 @@ COMMON_OBS = $(DEPFILES) $(CONFIG_OBS) $(YYOBJ) \
 	new-op.o \
 	print-utils.o \
 	ptid.o \
-	registry.o \
 	rsp-low.o \
 	run-time-clock.o \
 	selftest.o \
 	signals.o \
 	signals-state-save-restore.o \
-	thread-fsm.o \
 	vec.o \
 	version.o \
 	waitstatus.o \