CopyFFmpegDeps: Update variable name

FFmpeg_DLL_DIR does not exist anywhere else in the repository.
Evidently, the variable name was antiquated at some point, but it
continued to work here as a zombie.

Update the name and avoid copy issues.

CMakeModules/CopyYuzuFFmpegDeps.cmake

@@ -6,5 +6,5 @@ function(copy_yuzu_FFmpeg_deps target_dir)
     set(DLL_DEST "$<TARGET_FILE_DIR:${target_dir}>/")
     file(READ "${FFmpeg_PATH}/requirements.txt" FFmpeg_REQUIRED_DLLS)
     string(STRIP "${FFmpeg_REQUIRED_DLLS}" FFmpeg_REQUIRED_DLLS)
-    windows_copy_files(${target_dir} ${FFmpeg_DLL_DIR} ${DLL_DEST} ${FFmpeg_REQUIRED_DLLS})
+    windows_copy_files(${target_dir} ${FFmpeg_LIBRARY_DIR} ${DLL_DEST} ${FFmpeg_REQUIRED_DLLS})
 endfunction(copy_yuzu_FFmpeg_deps)