xcode generator: resolve QMAKE_INFO_PLIST from source dir

If QMAKE_INFO_PLIST is set, check if the file it
points to is located inside the project source dir
rather than the build dir.

Change-Id: I6fb176349dae8e841b5e2dfdb9f9cb87f51a1e76
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>