qextserialport: tell qmake to generate .pc file

Instead of creating our own .pc file with a patch, just tell qmake to
do it by adding create_pc to CONFIG.

This corrects the .pc file for Qt5. Previously, the include directory
was set to /usr/include, while for Qt5 it should be /usr/include/qt5.
This hasn't caused any autobuild failures since no other package uses
qextserialport.

Note, however, that the package is now called Qt5ExtSerialPort in Qt5,
while before it was qextserialport like in the Qt4 case.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>