stats: Make net config optional

To support reporters without network configuration, this commit
introduces the have_net_config flag to provide corresponding error
messages.

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/core/stats.h b/include/osmocom/core/stats.h
index ed461dd..9ee9f10 100644
--- a/include/osmocom/core/stats.h
+++ b/include/osmocom/core/stats.h
@@ -32,6 +32,8 @@
 	enum stats_reporter_type type;
 	char *name;
 
+	unsigned int have_net_config : 1;
+
 	/* config */
 	int enabled;
 	char *name_prefix;