Jolla: Mark IOS plugin as experimental

Signed-off-by: Jarkko Lehtoranta <jarkko.lehtoranta@jolla.com>
Jolla: Add install_dir/lib to the rpaths

This is needed for libicu, but also other additional 3rdparty libraries
can now be included directly under install_dir/lib.

Signed-off-by: Jarkko Lehtoranta <jarkko.lehtoranta@jolla.com>