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

Detected by Smatch
diff --git a/src/backtrace.c b/src/backtrace.c
index 8281fad..dfe0305 100644
--- a/src/backtrace.c
+++ b/src/backtrace.c
@@ -29,7 +29,7 @@
 
 #ifdef HAVE_EXECINFO_H
 #include <execinfo.h>
-void osmo_generate_backtrace()
+void osmo_generate_backtrace(void)
 {
 	int i, nptrs;
 	void *buffer[100];