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/fkeys.c

@@ -426,7 +426,7 @@ key_binding_is_builtin (struct key_binding *kb)
 }
 
 void
-key_init ()
+key_init (void)
 {
 	if (key_load_kbs () == 1)
 	{
@@ -1219,7 +1219,7 @@ key_dialog_load (GtkListStore *store)
 }
 
 void
-key_dialog_show ()
+key_dialog_show (void)
 {
 	GtkWidget *vbox, *box, *pane, *scroll;
 	GtkWidget *view, *xtext, *delete_button;

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;

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];

src/fe-gtk/urlgrab.c

@@ -187,7 +187,7 @@ populate_cb (char *urltext, gpointer userdata)
 }
 
 void
-url_opengui ()
+url_opengui (void)
 {
 	GtkWidget *vbox, *hbox, *view;
 	char buf[128];