Both CMAKE_FIND_ROOT_PATH_MODE_INCLUDE and
CMAKE_FIND_ROOT_PATH_MODE_LIBRARY are set to "ONLY" when cross
building to iOS, but appears that CMAKE_FIND_ROOT_PATH_MODE_PACKAGE
was overlooked.

This causes packages to be searched for in the host system as well,
which is incorrect and can lead to linking issues.

Set CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to "ONLY" as well.

CMAKE_FIND_ROOT_PATH_MODE_PROGRAM is not touched, because a user
might want to find programs / tools on the host system.