libgtk2: Fix pixbuf loaders cache file generation

After libgtk was bumped to 2.22.0 default loacation of
pixbuf loaders cache changed.
Use --update-cache flag for gdk-pixbuf-query-loaders
which does waht we need.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>