MINOR: ssl: add SSL_SERVER_LOCK label in threads.h

Amaury reported that the commit 3ce6eed ("MEDIUM: ssl: add a rwlock for
SSL server session cache") introduced some warning during compilation:

    include/haproxy/thread.h|411 col 2| warning: enumeration value 'SSL_SERVER_LOCK' not handled in switch [-Wswitch]

This patch fix the issue by adding the right entry in the switch block.

Must be backported where 3ce6eed is backported. (2.4 only for now)
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index 9982a76..bdabcac 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -442,6 +442,7 @@
 	case PROTO_LOCK:           return "PROTO";
 	case CKCH_LOCK:            return "CKCH";
 	case SNI_LOCK:             return "SNI";
+	case SSL_SERVER_LOCK:      return "SSL_SERVER";
 	case SFT_LOCK:             return "SFT";
 	case OTHER_LOCK:           return "OTHER";
 	case LOCK_LABELS:          break; /* keep compiler happy */