osdep/compiler: add mp_unused define

[llyyr]

Alternative to #14736

This is the supported way of doing this, and while (void)(x) works, there's no guarantee that it won't stop working in the future similar to how casting to void to silence unused-result warnings used to work in the past, but now they don't and likely will continue to not work.

[github-actions[bot]]

Download the artifacts for this pull request:

Windows

• mpv-i686-w64-mingw32
• mpv-x86_64-w64-mingw32
• mpv-x86_64-windows-msvc

macOS

• mpv-macos-12-intel
• mpv-macos-13-intel
• mpv-macos-14-arm

[llyyr]

Closing because there's no interest in this