FFmpeg: correct licenses install location

3rdparty/ffmpeg/ffmpeg-download.ps1.in

@@ -4,7 +4,7 @@ $output = "$PSScriptRoot\@OPENCV_BIN_INSTALL_PATH@\opencv_ffmpeg@OPENCV_DLLVERSI
 
 Write-Output ("=" * 120)
 try {
-    Get-content -Path "$PSScriptRoot\etc\licenses\ffmpeg-readme.txt" -ErrorAction 'Stop'
+    Get-content -Path "$PSScriptRoot\@OPENCV_LICENSES_INSTALL_PATH@\ffmpeg-readme.txt" -ErrorAction 'Stop'
 } catch {
     Write-Output "Refer to OpenCV FFmpeg wrapper readme notes about library usage / licensing details."
 }

3rdparty/ffmpeg/ffmpeg.cmake

@@ -40,3 +40,5 @@ if(OPENCV_INSTALL_FFMPEG_DOWNLOAD_SCRIPT)
   configure_file("${CMAKE_CURRENT_LIST_DIR}/ffmpeg-download.ps1.in" "${CMAKE_BINARY_DIR}/win-install/ffmpeg-download.ps1" @ONLY)
   install(FILES "${CMAKE_BINARY_DIR}/win-install/ffmpeg-download.ps1" DESTINATION "." COMPONENT libs)
 endif()
+
+ocv_install_3rdparty_licenses(ffmpeg license.txt readme.txt)