Use symlink to simplify Windows ffmpeg dependency

The FFmpeg dependency is downloaded from two separate zipfiles.

Symlink include/ to expose everything from a single directory, to
simplify the meson script.

PR #2952 <https://github.com/Genymobile/scrcpy/pull/2952>

cross_win64.txt

@@ -16,6 +16,5 @@ cpu = 'x86_64'
 endian = 'little'
 
 [properties]
-prebuilt_ffmpeg_shared = 'ffmpeg-4.3.1-win64-shared'
-prebuilt_ffmpeg_dev = 'ffmpeg-4.3.1-win64-dev'
+prebuilt_ffmpeg = 'ffmpeg-4.3.1-win64-shared'
 prebuilt_sdl2 = 'SDL2-2.0.18/x86_64-w64-mingw32'