rxrpc: Fix trace string

The trace symbol printer (__print_symbolic()) ignores symbols that map to
an empty string and prints the hex value instead.

Fix the symbol for rxrpc_cong_no_change to " -" instead of "" to avoid
this.

Fixes: b54a134a ("rxrpc: Fix handling of enums-to-string translation in tracing")
Signed-off-by: David Howells <dhowells@redhat.com>

include/trace/events/rxrpc.h

@@ -400,7 +400,7 @@ enum rxrpc_tx_point {
 	EM(rxrpc_cong_begin_retransmission,	" Retrans") \
 	EM(rxrpc_cong_cleared_nacks,		" Cleared") \
 	EM(rxrpc_cong_new_low_nack,		" NewLowN") \
-	EM(rxrpc_cong_no_change,		"") \
+	EM(rxrpc_cong_no_change,		" -") \
 	EM(rxrpc_cong_progress,			" Progres") \
 	EM(rxrpc_cong_retransmit_again,		" ReTxAgn") \
 	EM(rxrpc_cong_rtt_window_end,		" RttWinE") \