update_gtk_icon_cache: Fix for GTK4-only builds

update_gtk_icon_cache: Fix for GTK4-only builds

Try to execute both gtk-update-icon-cache and
gtk4-update-icon-cache after checking whether the
commands are available.

This attempts to match what gtk-icon-cache.bbclass is doing.

This fixes running update_gtk_icon_cache during do_rootfs
for an image that contains only GTK4 related packages.

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>