LICENSE: mention that uspp is GPL

LICENSE.md

@@ -49,6 +49,7 @@ Specifically, the GPL parts of FFmpeg are:
     - vf_stereo3d.c
     - vf_super2xsai.c
     - vf_tinterlace.c
+    - vf_uspp.c
     - vsrc_mptestsrc.c
 
 Should you, for whatever reason, prefer to use version 3 of the (L)GPL, then