gitignore: handle case where .vim-src is a symlink

I build Vim and Nvim from source and have both repos cloned locally. To prevent
vim-patch.sh from downloading the Vim source once again to /.vim-src/, I usually
use a symlink pointing to the already existing Vim repo.

The pattern "/.vim-src" works for both, a directory or a symlink.

.gitignore

@@ -29,7 +29,7 @@ compile_commands.json
 /.nvimlog
 
 # Generated by scripts/vim-patch.sh
-/.vim-src/
+/.vim-src
 
 # Generated by old (Vim) tests.
 /src/nvim/testdir/del