aacenc_tns: remove unused header

Thanks to @nevcairiel for pointing this one out.
Another thing which stopped msvc from compiling.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/aacenc_tns.c

@@ -25,7 +25,6 @@
  * @author Rostislav Pehlivanov ( atomnuker gmail com )
  */
 
-#include <strings.h>
 #include "aacenc.h"
 #include "aacenc_tns.h"
 #include "aactab.h"