Update formula to depends on 'sdl' package

It seems like this requirement has been missing from the package formula:

    [~] brew install --HEAD mpv
    ==> Cloning https://github.com/mpv-player/mpv.git
    Updating /Library/Caches/Homebrew/mpv--git
    ==> Generating VERSION file from Homebrew's git cache
    ==> ./configure --prefix=/usr/local/Cellar/mpv/HEAD --cc=cc --enable-macosx-bundle --disable-x11
    ==> make install
    ==> make osxbundle
    > copying binary
    > generating Info.plist
    > bundling dependencies
    mpv.app/Contents/MacOS/lib/libavutil.52.18.100.dylib uses library /usr/local/lib/libSDL-1.2.0.dylib which is not available anymore
    make: *** [osxbundle] Error 1
    
    READ THIS: https://github.com/mxcl/homebrew/wiki/troubleshooting