www/qt5-webengine: prepare for ffmpeg4

Add patch obtained from Arch Linux:
https://git.archlinux.org/svntogit/packages.git/tree/trunk/qtwebengine-ffmpeg4.patch?h=packages/qt5-webengine&id=421103a8798335f7788498e56b5eef3f33ecba26

PR: 227745
Reported by: jbeich