Don't rely on implicit rsa.h inclusion

With no-deprecated, some nested includes don't happen by default.
Reviewed-by: NRich Salz <rsalz@openssl.org>

fuzz/server.c

@@ -15,6 +15,7 @@
 
 #include <openssl/rand.h>
 #include <openssl/ssl.h>
+#include <openssl/rsa.h>
 #include "fuzzer.h"
 
 static const uint8_t kCertificateDER[] = {