include: fix missing extern in osmo_panic* declarations

This patch adds the missing extern to osmo_panic* declarations.
diff --git a/include/osmocom/core/panic.h b/include/osmocom/core/panic.h
index c5a8377..c28a844 100644
--- a/include/osmocom/core/panic.h
+++ b/include/osmocom/core/panic.h
@@ -5,7 +5,7 @@
 
 typedef void (*osmo_panic_handler_t)(const char *fmt, va_list args);
 
-void osmo_panic(const char *fmt, ...);
-void osmo_set_panic_handler(osmo_panic_handler_t h);
+extern void osmo_panic(const char *fmt, ...);
+extern void osmo_set_panic_handler(osmo_panic_handler_t h);
 
 #endif