MINOR: log: introduce ha_notice()

It's like ha_warning() or ha_alert() but with a NOTICE prefix.
diff --git a/include/proto/log.h b/include/proto/log.h
index 577e570..e127431 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -111,6 +111,12 @@
 	__attribute__ ((format(printf, 1, 2)));
 
 /*
+ * Displays the message on stderr with the date and pid.
+ */
+void ha_notice(const char *fmt, ...)
+	__attribute__ ((format(printf, 1, 2)));
+
+/*
  * Displays the message on <out> only if quiet mode is not set.
  */
 void qfprintf(FILE *out, const char *fmt, ...)