codec_libgav1.c: Remove #include <string.h>

After commit c64ea94, codec_libgav1.c no longer calls any function
declared in <string.h>. This can be verified by searching for "str" and
"mem" in this file. Include <stddef.h> instead (for NULL).

src/codec_libgav1.c

diff --git a/src/codec_libgav1.c b/src/codec_libgav1.c
index ec791cc..a529b5a 100644
--- a/src/codec_libgav1.c
+++ b/src/codec_libgav1.c
@@ -5,7 +5,7 @@
 
 #include "gav1/decoder.h"
 
-#include <string.h>
+#include <stddef.h>
 
 struct avifCodecInternal
 {