msdk: add support for open sourced MediaSDK

MediaSDK has been released as open source [1], but the directories
where it installs its files, are different from the binary only
distribution.

This patch adds to the libraries path the directory /lib. Also it
is defined in meson if the include directory has the mfx/ prefix,
something that is already handled in autotools.

1. https://github.com/Intel-Media-SDK/MediaSDK
2 jobs for oss-msdk in 2 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Manifest
passed manifest #47557

00:00:21

 
  Build
passed build fedora x86_64 #47558

00:02:31