qmake/Xcode: Mark "Qt Preprocess" build phase as always out of date

This build phase executes the qt_preprocess.mak makefile from Xcode
and should be triggered whenever any input of any rule in this makefile
is out of date.  It was not triggered when a file that's referenced in
a .qrc file was changed.

The Xcode project lacks those files as rule input, but the makefile
itself has its inputs correctly set up, and can be triggered always.

Pick-to: 6.2 5.15
Fixes: QTBUG-94995
Change-Id: Ida1349039bd6f23a300a610ecbde96f7cd35edb6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

qmake/generators/mac/pbuilder_pbx.cpp

@@ -873,6 +873,7 @@ ProjectBuilderMakefileGenerator::writeMakeParts(QTextStream &t)
           << "\t\t\t" << writeSettings("shellScript", "make -C " + IoUtils::shellQuoteUnix(Option::output_dir)
                                                       + " -f " + IoUtils::shellQuoteUnix(mkfile)) << ";\n"
           << "\t\t\t" << writeSettings("showEnvVarsInLog", "0") << ";\n"
+          << "\t\t\t" << writeSettings("alwaysOutOfDate", "1") << ";\n"
           << "\t\t};\n";
    }