yad: fix libgtk3 dependency

Make it depend explicitly on BR2_PACKAGE_LIBGTK3_X11 (the X11 backend
option) to avoid the checker from complaining.
Also add a comment about yad requiring the X11 backend otherwise it's
ambiguous if a user has libgtk3 enabled with broadway/wayland and the
comment shows up. Fixes:
http://autobuild.buildroot.net/results/84d/84d97bbcc0ea4f8eaa50dd25bf9ee8fdee4b937f/



Also cleanup some tiny whitespace in the package mk file.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>