buildscripts: disable vulkan in ffmpeg
buildscripts: disable vulkan in ffmpeg

Vulkan decoding is not only not supported on Android but
Google also deleted the respective header file so any
code that sets VK_ENABLE_BETA_EXTENSIONS will just fail to compile.
FFmpeg is affected so just disable it explicitly.

mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android