stats: Add osmo_ name prefix to identifiers

Since the the stat_item and stats functions and data types are meant
to be exported, they get an osmo_ prefix.

Sponsored-by: On-Waves ehf

[hfreyther: Prepended the enum values too. This was requested by
Jacob]
diff --git a/include/osmocom/vty/stats.h b/include/osmocom/vty/stats.h
index eaf854e..3851b4d 100644
--- a/include/osmocom/vty/stats.h
+++ b/include/osmocom/vty/stats.h
@@ -1,3 +1,3 @@
 #pragma once
 
-void stats_vty_add_cmds();
+void osmo_stats_vty_add_cmds();