Fix for memcpy() and strcmp() being undefined.

clang says: "s_cb.c:958:9: error: implicitly declaring library function
'memcpy'"

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>

apps/s_cb.c

@@ -112,6 +112,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>
+#include <string.h> /* for memcpy() and strcmp() */
 #define USE_SOCKETS
 #include "apps.h"
 #undef USE_SOCKETS