mesa: add tags file to gitignore

For ctags users like me.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>
diff --git a/.gitignore b/.gitignore
index b4f88f7..7db5639 100644
--- a/.gitignore
+++ b/.gitignore
@@ -34,6 +34,7 @@
 config.log
 config.status
 cscope*
+tags
 .scon*
 config.py
 build