gitignore: adpat to scanner and protocol path changes

Since commit 4c163b9b, wayland-scanner
is built in top builddir instead of src, and protocol files are
generated in protocol subdir instead of src.
Protocol files generated in the new path are already properly ignored
in the toplevel gitignore file.

Signed-off-by: Olivier Blin <olivier.blin@softathome.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>