libgtk3: rename patch

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
libgtk3: change method to build host-libgtk3

Change the way of building host-libgtk3, without a patch to reduce
dependencies.

Adapted from Eric Le Bihan branch:
https://github.com/elebihan/buildroot/tree/libgtk3-support

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
libgtk3: remove explicit disabling of xkb

Fix a build error if xkb support is explicitly disable.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>