qt: scripttools needs gui

The QtScriptTools library depends on the QtGui library, as can be seen
in the src/src.pro file of the Qt sources :

!contains(QT_CONFIG, no-gui):contains(QT_CONFIG, scripttools): SRC_SUBDIRS += src_scripttools

If the scripttools are enabled in the Qt configuration, but not the
gui, then the scripttools library is not built by Qt, and the
Buildroot build process fails with:

cp: cannot stat `/home/test/outputs/test-502/host/usr/sh4-unknown-linux-gnu/sysroot/usr/lib/libQtScriptTools.so.*': No such file or directory

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>