Use (void) for no parameters, not ()

This ensures we build with -Werror=missing-parameter-type.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

tests/assert-msg-test.c

@@ -1,6 +1,6 @@
 #include <glib.h>
 
-int main()
+int main(int argc, char **argv)
 {
     g_assert(42 < 0);
     return 0;