ignore symbol files on OS X (only for debugging, breaks git-add -A)

To apply this patch, use:
curl http://cr.i3wm.org/patch/336/raw.patch | git am

@@ -6,6 +6,7 @@ include/all.h.pch

 *.swp

 *.gcda

 *.gcno

+*.dSYM

 test.commands_parser

 test.config_parser

 testcases/MYMETA.json