net/freerdp: ffmpeg 6 support

Fix build with FFMPEG option on with ffmpeg 6.

1. Error "AV_CODEC_CAP_TRUNCATED", upstream issue with pull request: https://github.com/FreeRDP/FreeRDP/issues/8686
2. Error "AVPicture", upstream pull request: https://github.com/FreeRDP/FreeRDP/pull/8941

Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D40492