Fix enable-sctp

Commit b99fe5f4 broke SCTP. This fixes it again.
Reviewed-by: NStephen Henson <steve@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4244)

test/handshake_helper.c

@@ -16,6 +16,7 @@
 #include <openssl/srp.h>
 #endif
 
+#include "internal/sockets.h"
 #include "internal/nelem.h"
 #include "handshake_helper.h"
 #include "testutil.h"