LICENSE.md: add *_rect filters

Found-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

LICENSE.md

@@ -27,9 +27,11 @@ Specifically, the GPL parts of FFmpeg are:
     - `vf_blackframe.c`
     - `vf_boxblur.c`
     - `vf_colormatrix.c`
+    - `vf_cover_rect.c`
     - `vf_cropdetect.c`
     - `vf_delogo.c`
     - `vf_eq.c`
+    - `vf_find_rect.c`
     - `vf_fspp.c`
     - `vf_geq.c`
     - `vf_histeq.c`