Fix libplacebo/ffmpeg builds on NDKv26

This fixes a confusing error related to Vulkan headers when compiling libplacebo/ffmpeg - for example in mpv - specifically on Android with NDK v26. The reason for this is some misplaced Vulkan headers in this NDK, see https://github.com/android/ndk/issues/1974. But in reality we should just turn off Vulkan decode support entirely for these libraries as Google recommends not to use these extensions.