BUILD: backend: silence a build warning when not using ssl

Since recent commit ee94a6cfc1 ("MINOR: backend: extract conn reuse
from connect_server()") a build warning "set but not used" on the
"reuse" variable is emitted, because indeed the variable is now only
checked when SSL is in use. Let's just mark it as such.

src/backend.c

@@ -1779,7 +1779,7 @@ int connect_server(struct stream *s)
 	struct connection *srv_conn = NULL;
 	struct server *srv;
 	int reuse_mode;
-	int reuse = 0;
+	int reuse __maybe_unused = 0;
 	int init_mux = 0;
 	int err;
 	struct sockaddr_storage *bind_addr = NULL;