graphics/gdk-pixbuf2: add trigger to run gdk-pixbuf-query-loaders

gdk-pixbuf2 uses a cache of its loadable modules, which is updated by
${RREFIX}/bin/gdk-pixbuf-query-loaders and stored in
${PREFIX}/lib/gdk-pixbuf-2.0/${GTK2_VERSION}/loaders.

Remove the pre-existing @exec and @postexec commands in the ports
providing such modules, and add a trigger in graphics/gdk-pixbuf2
to do this automatically.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D34679