qmake: add ProStringList::join(QChar)

Same reasoning as for 68e04c3ac148bcbe71f2deeb7288563f6cdbcab5 applies.

Adding the overload was easier than to teach a Perl script to distinguish
between QStringList and ProStringList instances...

Change-Id: I6de6ecf21fdad135ac213b5c794927a9bc120a92
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/cbf447069cfeb799ff5e09902be065d77f2e7707)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>