qtscript: Check for QtGui before building the examples

qsdbg and marshal examples need QtGui module. If QtGui is not installed
they fail to compile.

Change-Id: I1af6a8b39a0848c8208fe94a26ee57409dfd9b41
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>