LICENSE: Document all GPL files

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

LICENSE

@@ -14,6 +14,14 @@ configure to activate them. In this case, Libav's license changes to GPL v2+.
 Specifically, the GPL parts of Libav are
 
 - the X11 grabber in libavdevice/x11grab.c
+- the texi2pod.pl tool
+- the following filters in libavfilter:
+    - vf_blackframe.c
+    - vf_boxblur.c
+    - vf_cropdetect.c
+    - vf_delogo.c
+    - vf_hqdn3d.c
+    - vf_yadif.c
 
 There are a handful of files under other licensing terms, namely: