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/textgui.c

@@ -486,7 +486,7 @@ pevent_hlist_treeview_new (GtkWidget *box)
 }
 
 void
-pevent_dialog_show ()
+pevent_dialog_show (void)
 {
 	GtkWidget *vbox, *hbox, *wid, *pane, *preview_pane;
 	XTextColor xtext_palette[XTEXT_COLS];