use parking_lot::Once in ffmpeg

I'm getting deprecation warnings for std::sync::ONCE_INIT, and I'm
not sure when std::sync::Once::new() became a const fn. Just as easy to
switch to parking_lot.

ffmpeg/Cargo.toml

@@ -12,6 +12,7 @@ path = "lib.rs"
 [dependencies]
 libc = "0.2"
 log = { version = "0.4", features = ["release_max_level_info"] }
+parking_lot = { version = "0.9", features = [] }
 
 [build-dependencies]
 cc = "1.0"