BUILD: ssl: fix build of backend without ssl

Commit 522eea7 ("MINOR: ssl: Handle sending early data to server.") added
a dependency on SRV_SSL_O_EARLY_DATA which only exists when USE_OPENSSL
is defined (which is probably not the best solution) and breaks the build
when ssl is not enabled. Just add an ifdef USE_OPENSSL around the block
for now.
diff --git a/src/backend.c b/src/backend.c
index ca064b6..c8c2b9d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1209,6 +1209,7 @@
 
 	err = si_connect(&s->si[1]);
 
+#ifdef USE_OPENSSL
 	if (!reuse && cli_conn && srv &&
 	    (srv->ssl_ctx.options & SRV_SSL_O_EARLY_DATA) &&
 		    (cli_conn->flags & CO_FL_EARLY_DATA) &&
@@ -1217,6 +1218,7 @@
 		srv_conn->flags &= ~(CO_FL_SSL_WAIT_HS | CO_FL_WAIT_L6_CONN);
 		srv_conn->flags |= CO_FL_EARLY_SSL_HS;
 	}
+#endif
 
 	if (err != SF_ERR_NONE)
 		return err;