gitignore: Ignore 'tags' file generated by ctags

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
diff --git a/.gitignore b/.gitignore
index ef03b17..74a5496 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,6 +10,7 @@
 # cscope
 cscope.*
 ncscope.*
+tags
 TAGS
 
 # git files that we don't want to ignore even it they are dot-files