package/qt5base: only build libgtk3 support for X11 backend

libgtk3 can conditionally be build with or without the X11 backend
unlike libgtk2. If libgtk3 is enabled and the X11 backend is not enabled
for BR2_PACKAGE_QT5_VERSION_LATEST, QT_EGL_NO_X11 or
MESA_EGL_NO_X11_HEADERS needs to be defined.

The proper way is to patch Qt to use pkg-config to fetch the correct
flags [1], or to add it to arch.conf like is done in commit 7d286be4.
For now, however, just disable libgtk3 support unless the X11 backend
is used.

Fixes http://autobuild.buildroot.net/results/6f3c2fcb0a44cd13ef4c73822d4d08f923291099

[1] https://codereview.qt-project.org/#/c/198906/



Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Cc: Brent Sink <brents_3@hotmail.com>
Cc: Naoki Matsumoto <n-matsumoto@melcoinc.co.jp>
[Arnout:
- extend commit message
- improve comment
- also pass -no-gtk if libgtk3 is not selected]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>