qt5serialbus: examples need qt5 widgets module

qt5serialbus: examples need qt5 widgets module

Fixes [1]:

  make[2]: Entering directory '.../build/qt5serialbus-5.11.1/examples'
  Some of the required modules (qtHaveModule(widgets)) are not available.
  Skipped.
  [...]
  cp -dpfr .../host/mips64el-buildroot-linux-gnu/sysroot/usr/lib/qt/examples/serialbus .../target/usr/lib/qt/examples/
  cp: cannot stat '.../host/mips64el-buildroot-linux-gnu/sysroot/usr/lib/qt/examples/serialbus': No such file or directory

[1] http://autobuild.buildroot.net/results/147809b5f8758af935bee48c0fc83fd86a8509e9

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