Makefile: add a symbol link tags for ctags

Signed-off-by: Du Huanpeng <u74147@gmail.com>
diff --git a/Makefile b/Makefile
index 24503ac..cecb8fa 100644
--- a/Makefile
+++ b/Makefile
@@ -1272,6 +1272,7 @@
 tags ctags:
 		ctags -w -o ctags `$(FIND) $(FINDFLAGS) $(TAG_SUBDIRS) \
 						-name '*.[chS]' -print`
+		ln -s ctags tags
 
 etags:
 		etags -a -o etags `$(FIND) $(FINDFLAGS) $(TAG_SUBDIRS) \