macOS package: Increase image size for dmg package

Increase to 300M to allow to build universal releases.
This is the maximum size of the release image, the final
file will be only at the usual compressed size.

(cherry picked from commit 6a8dcbee0f3925af7ace8afb0b1a5b58c69f354f)
Signed-off-by: David Fuhrmann <dfuhrmann@videolan.org>