Makefile: include deps from tools directory

This ensures the tools are rebuilt when necessary.  Specifically,
lavfi-showfiltfmts was sometimes not rebuilt causing spurious test
failures.

Signed-off-by: Mans Rullgard <mans@mansr.com>

Makefile

@@ -92,6 +92,8 @@ tools/%$(EXESUF): tools/%.o
 tools/%.o: tools/%.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) -c $(CC_O) $<
 
+-include $(wildcard tools/*.d)
+
 ffplay.o: CFLAGS += $(SDL_CFLAGS)
 
 VERSION_SH  = $(SRC_PATH_BARE)/version.sh