sae: fix missing reallocarray definition

Building on GCC 8 resulted in this compiler error.

src/sae.c:107:25: error: implicit declaration of function 'reallocarray';
	did you mean 'realloc'? [-Werror=implicit-function-declaration]
	sm->rejected_groups = reallocarray(NULL, 2, sizeof(uint16_t));

src/sae.c

@@ -24,6 +24,8 @@
 #include <config.h>
 #endif
 
+#include <stdlib.h>
+
 #include <ell/ell.h>
 
 #include "src/missing.h"