Fix compiler warnings with GCC 8.1.

GCC 8.1 added some new warnings, including warning about parentheses
with no effect in variable declarations.  GTK2 headers have a few of
these, which produce a lot of warnings in projects using GTK2.

The warnings look like:
/usr/include/gtk-2.0/gtk/gtkfilechooserbutton.h:59:8: warning:
unnecessary parentheses in declaration of ‘__gtk_reserved1’ [-Wparentheses]
   void (*__gtk_reserved1);
        ^

Removing the parentheses is harmless and fixes the warnings.

gtk/gtkfilechooserbutton.h

@@ -56,13 +56,13 @@ struct _GtkFileChooserButtonClass
 
   void (* file_set) (GtkFileChooserButton *fc);
 
-  void (*__gtk_reserved1);
-  void (*__gtk_reserved2);
-  void (*__gtk_reserved3);
-  void (*__gtk_reserved4);
-  void (*__gtk_reserved5);
-  void (*__gtk_reserved6);
-  void (*__gtk_reserved7);
+  void *__gtk_reserved1;
+  void *__gtk_reserved2;
+  void *__gtk_reserved3;
+  void *__gtk_reserved4;
+  void *__gtk_reserved5;
+  void *__gtk_reserved6;
+  void *__gtk_reserved7;
 };
 
 

gtk/gtkstatusicon.h

@@ -73,8 +73,8 @@ struct _GtkStatusIconClass
                                      gboolean        keyboard_mode,
                                      GtkTooltip     *tooltip);
 
-  void (*__gtk_reserved1);
-  void (*__gtk_reserved2);
+  void *__gtk_reserved1;
+  void *__gtk_reserved2;
 };
 
 GType                 gtk_status_icon_get_type           (void) G_GNUC_CONST;