Fix clang -Wstrict-prototypes warnings

This trivial diff gets rid of noisy warnings such as
```
./src/common/ignore.c:274:13: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  274 | ignore_load ()
      |             ^
      |              void
```
when building with modern clang. This is cosmetic since these functions
are actually prototyped in a header, so if the header is in scope it's
not possible to call them with an arbitrary number of arguments.

src/fe-gtk/ignoregui.c

@@ -336,7 +336,7 @@ ignore_stats_entry (GtkWidget * box, char *label, int value)
 }
 
 void
-ignore_gui_open ()
+ignore_gui_open (void)
 {
 	GtkWidget *vbox, *box, *stat_box, *frame;
 	GtkWidget *view;