Fix type issue for compilation targeting windows

[Nutigoodpipes]

Issue

When trying to compile for the target x86_64-pc-windows-gnu, it gives the following error:

localhost% PKG_CONFIG_ALLOW_CROSS=1 cargo build --release --target=x86_64-pc-windows-gnu
   Compiling ffmpeg-sys v4.3.2 (/home/_/Code/rust-ffmpeg-sys)
error[E0587]: type has conflicting packed and align representation hints
    --> /home/_/Code/rust-ffmpeg-sys/target/x86_64-pc-windows-gnu/release/build/ffmpeg-sys-b96fa4f58482532a/out/bindings.rs:1930:1
     |
1930 | / pub union __mingw_ldbl_type_t {
1931 | |     pub x: u128,
1932 | |     pub lh: __mingw_ldbl_type_t__bindgen_ty_1,
1933 | | }
     | |_^

It seems that this is a bindgen (or rust) related issue, but this fix does it for me. Not really sure if this type should be blocklisted instead, but opaque may be a less breaking solution.

Environment

I'm actually compiling from a rustup installed stable toolchain on an archlinux system.