multimedia/mpv: force rebuild after r433759

$ mpv --no-config /path/to/foo.mp4
ffmpeg library versions:

libavutil       55.34.100 (runtime 55.34.101)
libavcodec      57.64.101
libavformat     57.56.100 (runtime 57.56.101)
libswscale      4.2.100
libavfilter     6.65.100
libswresample   2.3.100

ffmpeg version: 3.2.3

mpv was compiled against a different version of FFmpeg/Libav than the shared
library it is linked against. This is most likely a broken build and could
result in misbehavior and crashes.

mpv does not support this configuration and will not run - rebuild mpv instead.

Exiting... (Fatal error)