top .gitignore: ignore ctags tag files

Change-Id: I64538525626688b0cd1eda579294e547a2e40b30
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-on: http://openocd.zylin.com/1971
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

.gitignore

@@ -95,3 +95,6 @@ TAGS
 
 # CScope database files
 *cscope.out
+
+# ctags tag files
+tags