build: Allow FFMPEG_PLATFORM_LIBRARIES to be overridden by the user (#4915)

cmake/dependencies/ffmpeg.cmake

@@ -136,6 +136,7 @@ else()
     message(STATUS "Using user-specified FFmpeg binaries at ${FFMPEG_PREPARED_BINARIES}")
 
     # Set platform-specific libraries
+    if(NOT DEFINED FFMPEG_PLATFORM_LIBRARIES)
         if(WIN32)
             set(FFMPEG_PLATFORM_LIBRARIES mfplat ole32 strmiids mfuuid vpl)
         elseif(FREEBSD)
@@ -143,6 +144,7 @@ else()
         elseif(UNIX AND NOT APPLE)
             set(FFMPEG_PLATFORM_LIBRARIES numa va va-drm va-x11 X11)
         endif()
+    endif()
 
     # Set base FFmpeg libraries (always required)
     set(FFMPEG_LIBRARIES