shell: Don't avoid parameter list

Meh, C ...

src/shell-app-system.c

@@ -182,7 +182,7 @@ shell_app_system_finalize (GObject *object)
  * Return Value: (transfer none): The global #ShellAppSystem singleton
  */
 ShellAppSystem *
-shell_app_system_get_default ()
+shell_app_system_get_default (void)
 {
   static ShellAppSystem *instance = NULL;
 

src/shell-app-usage.c

@@ -996,7 +996,7 @@ on_enable_monitoring_key_changed (GSettings     *settings,
  * Return Value: (transfer none): The global #ShellAppUsage instance
  */
 ShellAppUsage *
-shell_app_usage_get_default ()
+shell_app_usage_get_default (void)
 {
   static ShellAppUsage *instance;
 

src/shell-util.c

@@ -165,7 +165,7 @@ shell_util_format_date (const char *format,
  */
 /* Copied from gtkcalendar.c */
 int
-shell_util_get_week_start ()
+shell_util_get_week_start (void)
 {
   int week_start;
 #ifdef HAVE__NL_TIME_FIRST_WEEKDAY

src/shell-window-tracker.c

@@ -865,7 +865,7 @@ shell_startup_sequence_create_icon (ShellStartupSequence *sequence, guint size)
  * Return Value: (transfer none): The global #ShellWindowTracker instance
  */
 ShellWindowTracker *
-shell_window_tracker_get_default ()
+shell_window_tracker_get_default (void)
 {
   static ShellWindowTracker *instance;