avcodec/noise_bsf: Remove unnecessary headers

With 33d18982, the commit introducing
the new packet-based bsf API, a new buffer was no longer allocated
directly, but via av_new_packet(), so that libavutil/mem.h was no longer
needed.

Moreover since commit dc99ee6b
av_packet_make_writable() is employed which copies the data in case it
is unavoidable so that string.h is no longer used (it was used for
memcpy()).
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/noise_bsf.c

@@ -19,13 +19,11 @@
  */
 
 #include <stdlib.h>
-#include <string.h>
 
 #include "avcodec.h"
 #include "bsf.h"
 
 #include "libavutil/log.h"
-#include "libavutil/mem.h"
 #include "libavutil/opt.h"
 
 typedef struct NoiseContext {