ppd-cache.c: Check for urf-supported if image/urf is found

Some devices have `image/urf` in `document-format-supported`, but is
missing `urf-supported` if AirPrint support is turned off, which breaks
PPD generation.

Check for attribute `urf-supported` when we are about to decide whether
the printer uses AirPrint, so in case the device supports another
driverless standard, we can use it for PPD generation.

Fixes [Fedora issue](https://bugzilla.redhat.com/show_bug.cgi?id=2263053)

cups/ppd-cache.c

@@ -3435,7 +3435,7 @@ _ppdCreateFromIPP2(
 
   if ((attr = ippFindAttribute(supported, "document-format-supported", IPP_TAG_MIMETYPE)) != NULL)
   {
-    is_apple = ippContainsString(attr, "image/urf");
+    is_apple = ippContainsString(attr, "image/urf") && (ippFindAttribute(supported, "urf-supported", IPP_TAG_KEYWORD) != NULL);
     is_pdf   = ippContainsString(attr, "application/pdf");
     is_pwg   = ippContainsString(attr, "image/pwg-raster") && !is_apple;