build: Mark private functions as static

Fixes the following error
../src/window/picplanner-window.c:247:1: error: no previous declaration for ‘picplanner_update_date_time’ [-Werror=missing-declarations]
  247 | picplanner_update_date_time (PicplannerWindow *window)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

src/picplanner-application.c

@@ -39,7 +39,7 @@ struct _PicplannerApplication
 G_DEFINE_TYPE (PicplannerApplication, picplanner_application, ADW_TYPE_APPLICATION)
 
 
-void
+static void
 first_run_set_timezone (PicplannerApplication *application)
 {
   gint64 timezone;

src/window/picplanner-window.c

@@ -246,7 +246,7 @@ picplanner_set_location (double latitude, double longitude, PicplannerWindow *wi
  * Changing the date_time variable after a user input was recognized.
  * Afterwards, emit a signal to start the calculations.
  */
-void
+static void
 picplanner_update_date_time (PicplannerWindow *window)
 {
   int year, month, day, hour, minute;