Missing dependencies for resources under XCode

When building Qt applications using XCode, the Qt resource files are not rebuilt automatically if the resource is updated.

This is due to the qmake xcode project generator, which doesn't include
the dependencies in qt_preprocess.mak file.

Current workarounds to force it to be built correctly include
deleting qrc_webcontent.cpp, touching webcontent.qrc or performing a
complete clean and build.

Building with qmake -spec macx-g++ and make handles the dependency
correctly.