Add distclean target

And only clean cscope.out output when doing that. I'm getting
tired of having to re-do cscope for no real reason.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/Makefile b/Makefile
index d10123e..781cdec 100644
--- a/Makefile
+++ b/Makefile
@@ -262,7 +262,10 @@
 	$(QUIET_LINK)$(CC) $(LDFLAGS) $(CFLAGS) -o $@ $(T_LFSR_TEST_OBJS) $(LIBS) $(LDFLAGS)
 
 clean: FORCE
-	-rm -f .depend $(FIO_OBJS) $(GFIO_OBJS) $(OBJS) $(T_OBJS) $(PROGS) $(T_PROGS) core.* core gfio FIO-VERSION-FILE config-host.mak config-host.h cscope.out *.d
+	-rm -f .depend $(FIO_OBJS) $(GFIO_OBJS) $(OBJS) $(T_OBJS) $(PROGS) $(T_PROGS) core.* core gfio FIO-VERSION-FILE *.d config-host.mak config-host.h
+
+distclean: clean FORCE
+	@rm -f cscope.out
 
 cscope:
 	@cscope -b -R