BUILD/MINOR: cli: fix a build warning introduced by last commit

Commit 35b1b48 ("MINOR: cli: make "show fd" report the mux and mux_ctx
pointers when available") introduced an accidental build warning due to
a missing const statement.
diff --git a/src/cli.c b/src/cli.c
index c9f2067..b7d428d 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -777,7 +777,7 @@
 		struct listener *li = NULL;
 		struct server *sv = NULL;
 		struct proxy *px = NULL;
-		struct mux_ops *mux = NULL;
+		const struct mux_ops *mux = NULL;
 		void *ctx = NULL;
 		uint32_t conn_flags = 0;