Makefile: Include dependencies for test tools, too

This makes seek_test to be rebuilt when its dependencies
has changed. The changes to the dependencies didn't usually matter
in practice, but the introduction of side data in AVPacket required
a recompilation.
Signed-off-by: NMartin Storsjö <martin@martin.st>

Makefile

@@ -87,6 +87,7 @@ tools/%.o: tools/%.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) -c $(CC_O) $<
 
 -include $(wildcard tools/*.d)
+-include $(wildcard tests/*.d)
 
 ffplay.o: CFLAGS += $(SDL_CFLAGS)