qt5tools: qtdiag needs BR2_PACKAGE_QT5BASE_GUI

Fixes [1]:

  /usr/bin/make -j4 -C /home/buildroot/autobuild/run/instance-3/output/build/qt5tools-5.5.1/src/qtdiag
  make[2]: *** No targets specified and no makefile found.  Stop.

[1] http://autobuild.buildroot.net/results/4ee/4eee2a7cee8fce63b0419668da8bd55b3cfd00af

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/qt5/qt5tools/Config.in

@@ -26,6 +26,7 @@ config BR2_PACKAGE_QT5TOOLS_PIXELTOOL
 
 config BR2_PACKAGE_QT5TOOLS_QTDIAG
 	bool "qtdiag"
+	select BR2_PACKAGE_QT5BASE_GUI
 	help
 	  Compile and install the qtdiag program.