package/gtkmm3: reorder select/depends

package/gtkmm3: reorder select/depends

>From [1]:
"Even though the ordering has absolutely no consequences in Kconfig, it
is not logical (when reading). It is more logical and far easier to
understand when depends come first, followed by the selects."

Also, the Config.in exemple in the manual suggest to use this coding
style [2].

[1] http://lists.busybox.net/pipermail/buildroot/2015-October/142955.html
[2] http://buildroot.uclibc.org/downloads/manual/manual.html#writing-rules-config-in

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>