update_gtk_immodules_cache: update for multilib

update_gtk_immodules_cache: update for multilib

Postinstall script update_gtk_immodules_cache calls
${bindir}/gtk-query-immodules-${version}. When multilib is enabled, both
packages foo and lib32-foo call ${bindir}/gtk-query-immodules-${version}
and one of them will fail to run obviously.

Duplicate install files gtk-query-immodules-${version} to ${libexecdir}
with ${MLPREFIX}. And update update_gtk_immodules_cache calls proper
binary.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>