configure.ac: Remove unused libplist++ requirement
configure.ac: Remove unused libplist++ requirement

libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice