fix gtk_widget_get_requisition deprecation warning in GTK3

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

shortcuts.c

@@ -169,10 +169,16 @@ sc_adjust_window(girara_session_t* session, girara_argument_t* argument,
             GTK_SCROLLED_WINDOW(session->gtk.view));
 
         if (vscrollbar != NULL) {
+            int scroll_width;
+#if (GTK_MAJOR_VERSION == 3)
+            gtk_widget_get_preferred_width(GTK_WIDGET(vscrollbar), NULL, &scroll_width);
+#else
           GtkRequisition requisition;
           gtk_widget_get_requisition(vscrollbar, &requisition);
-          if (0 < requisition.width && (unsigned)requisition.width < width) {
-            width -= requisition.width;
+          scroll_width = requisition.width;
+#endif
+          if (0 < scroll_width && (unsigned)scroll_width < width) {
+            width -= scroll_width;
             scale = (double)(width - (pages_per_row - 1) * padding) /
                     (double)(pages_per_row * cell_width);
             zathura_document_set_scale(zathura->document, scale);