[QScintilla] Missing include dir in python bindings build

[Antonio Rojas]

Hi,
 When using sip5, the build of python bindings is failing here because the QtWidgets include path is missing from the build flags:

g++ -c -pipe -O2 -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fno-exceptions -Wall -W -D_REENTRANT -fPIC -DPy_LIMITED_API=0x03040000 -DSIP_PROTECTED_IS_PUBLIC -Dprotected=public -DQT_NO_EXCEPTIONS -DQT_NO_DEBUG -DQT_PLUGIN -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -I.. -I../../Qt4Qt5 -isystem /usr/include/python3.8 -isystem /usr/include/qt -isystem /usr/include/qt/QtGui -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o sipQsciQsciCommandSet.o sipQsciQsciCommandSet.cpp
In file included from /build/qscintilla/src/QScintilla-2.11.4/Python/sip/qsciprinter.sip:26:
../../Qt4Qt5/Qsci/qsciprinter.h:29:10: fatal error: qprinter.h: No such file or directory
   29 | #include <qprinter.h>
      |          ^~~~~~~~~~~~


It seems the generated qmake file is missing a 'QT += widgets' line.