get rid of non-ANSI function declarations missing (void)

Detected by Smatch
diff --git a/include/osmocom/core/backtrace.h b/include/osmocom/core/backtrace.h
index 5a8a816..1ed089a 100644
--- a/include/osmocom/core/backtrace.h
+++ b/include/osmocom/core/backtrace.h
@@ -1,6 +1,6 @@
 #ifndef _OSMO_BACKTRACE_H_
 #define _OSMO_BACKTRACE_H_
 
-void osmo_generate_backtrace();
+void osmo_generate_backtrace(void);
 
 #endif