libgimpwidgets: get rid of build warning.

This fixes this warning (as appeared with commit ad8b47bff7):

[1/235] Compiling C object libgimpwidgets/libgimpwidgets-3.0.so.0.9900.17.p/gimpdialog.c.o
../../../../../../../dev/src/gimp/libgimpwidgets/gimpdialog.c:774:1: warning: ‘gimp_dialog_set_title_bar_theme’ defined but not used [-Wunused-function]
  774 | gimp_dialog_set_title_bar_theme (GtkWidget *dialog)

We could either put the whole `gimp_dialog_set_title_bar_theme()` declaration,
definition and usage into #ifdef, or only the implementation (making the
function a no-op on non-Windows platforms). I chose the former. There was some
discussion that maybe some implementation may happen later for other platforms,
but until then, no need to call it needlessly (even more as we don't know when
any theoretical other implementation would happen).

libgimpwidgets/gimpdialog.c

@@ -96,7 +96,9 @@ static void       gimp_dialog_close               (GtkDialog    *dialog);
 static void       gimp_dialog_response            (GtkDialog    *dialog,
                                                    gint          response_id);
 
+#ifdef G_OS_WIN32
 static void       gimp_dialog_set_title_bar_theme (GtkWidget    *dialog);
+#endif
 
 G_DEFINE_TYPE_WITH_PRIVATE (GimpDialog, gimp_dialog, GTK_TYPE_DIALOG)
 
@@ -770,10 +772,10 @@ gimp_dialogs_show_help_button (gboolean  show)
   show_help_button = show ? TRUE : FALSE;
 }
 
+#ifdef G_OS_WIN32
 void
 gimp_dialog_set_title_bar_theme (GtkWidget *dialog)
 {
-#ifdef G_OS_WIN32
   HWND             hwnd;
   gboolean         use_dark_mode = FALSE;
   GdkWindow       *window        = NULL;
@@ -810,5 +812,5 @@ gimp_dialog_set_title_bar_theme (GtkWidget *dialog)
       gdk_window_hide (window);
       gdk_window_show (window);
     }
-#endif
 }
+#endif