Hopefully address the issue with REQUIRED_VARS for FindFFMPEG.cmake in cmake earlier than 3.18

Properly addresses issue #121 (and hopefully discussion #140)

Signed-off-by: Alexis Maiquez <almamu@almamu.com>

CMakeModules/FindFFMPEG.cmake

@@ -24,6 +24,7 @@ include(FindPackageHandleStandardArgs)
         FOUND_VAR FFMPEG_FOUND
         VERSION_VAR FFMPEG_VERSION
         HANDLE_COMPONENTS
+        REQUIRED_VARS
     )
 
 if(FFMPEG_LIBRARIES AND FFMPEG_INCLUDE_DIR)