hash: Avoid including stdlib.h

Author: real-or-random

src/hash.h

@@ -7,7 +7,7 @@
 #ifndef SECP256K1_HASH_H
 #define SECP256K1_HASH_H
 
-#include <stdlib.h>
+#include <stddef.h>
 #include <stdint.h>
 
 typedef struct {

src/hash_impl.h

@@ -10,7 +10,7 @@
 #include "hash.h"
 #include "util.h"
 
-#include <stdlib.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <string.h>