main: Mark g_set_application_name for translation

According to [1], it should be marked for translation.

[1] https://developer.gnome.org/glib/stable/glib-Miscellaneous-Utility-Functions.html#g-set-application-name

src/main.c

@@ -33,7 +33,7 @@ main (int   argc,
   bind_textdomain_codeset (GETTEXT_PACKAGE, "UTF-8");
   textdomain (GETTEXT_PACKAGE);
 
-  g_set_application_name ("Calendar");
+  g_set_application_name (_("Calendar"));
 
   app = gcal_application_new ();