BUILD: mux-spop: fix build failure on gcc 4-10 and clang

A label at end of block was added in mux_spop.c in function
spop_conn_update_timeout() by commit 7e1bb7283b ("MEDIUM: mux-spop:
Introduce the SPOP multiplexer"). This is normally not permitted,
so gcc-4 to 10 and clang whine about it:

    CC      src/mux_spop.o
  src/mux_spop.c: In function 'spop_conn_update_timeout':
  src/mux_spop.c:899:2: error: label at end of compound statement
    899 |  leave:
        |  ^~~~~

Let's just add a return there to make the compiler happy. No backport
is needed.

src/mux_spop.c

@@ -897,6 +897,7 @@ static void spop_conn_update_timeout(struct spop_conn *spop_conn)
 	task_queue(spop_conn->task);
 	TRACE_LEAVE(SPOP_EV_SPOP_CONN_WAKE, spop_conn->conn, 0, 0, (size_t[]){spop_conn->task->expire});
  leave:
+	return;
 }
 
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