license: Mention that vf_interlace is GPL, not LGPL

(cherry picked from commit 9e8bbe7d4d)
Signed-off-by: Diego Biurrun <diego@biurrun.de>

LICENSE

@@ -21,6 +21,7 @@ Specifically, the GPL parts of Libav are
     - vf_cropdetect.c
     - vf_delogo.c
     - vf_hqdn3d.c
+    - vf_interlace.c
 
 Should you, for whatever reason, prefer to use version 3 of the (L)GPL, then
 the configure parameter --enable-version3 will activate this licensing option