ffmpeg: Depends on xz for Linuxbrew
ffmpeg: Depends on xz for Linuxbrew

Depends on xz for liblzma.so.5
Fix ffmpeg: error while loading shared libraries: liblzma.so.5

Closes Linuxbrew/homebrew-core#95.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

maxim-belkin/homebrew-core,forevergenin/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,forevergenin/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,Linuxbrew/homebrew-core