Force linking against user32 if dxva2 is enabled.

Fixes compilation with vfwcap disabled.
Reported and tested by Bernhard Döbler, programmer bardware de

Reviewed-by: Matt Oliver

configure

@@ -5373,7 +5373,7 @@ if test $target_os = "haiku"; then
 fi
 
 enabled_all dxva2 dxva2api_cobj CoTaskMemFree &&
-    prepend ffmpeg_libs $($ldflags_filter "-lole32") &&
+    prepend ffmpeg_libs $($ldflags_filter "-lole32" "-luser32") &&
     enable dxva2_lib
 
 ! enabled_any memalign posix_memalign aligned_malloc &&