Fixes the following compile error

In file included from simu.c:14:0:
../../support/include/sockaddr.h: In function 'nfs_compare_sockaddr':
../../support/include/sockaddr.h:230:13: error: 'NULL' undeclared (first use in this function)
  if (sa1 == NULL || sa2 == NULL)
             ^
../../support/include/sockaddr.h:230:13: note: each undeclared identifier is reported only once for each function it appears in

--- support/include/sockaddr.h
+++ support/include/sockaddr.h
@@ -28,6 +28,7 @@
 #include <libio.h>
 #endif
 #include <stdlib.h>
+#include <stddef.h>
 #include <stdbool.h>
 #include <sys/socket.h>
 #include <netinet/in.h>