vf_colormatrix: Drop unused strings.h header

It already uses av_strcasecmp.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_colormatrix.c

@@ -28,7 +28,6 @@
  * adds an option to use scaled or non-scaled coefficients, and more...
  */
 
-#include <strings.h>
 #include <float.h>
 #include "avfilter.h"
 #include "formats.h"