Fix the build with cups < 2.3

PPD_CUSTOM_UNKNOWN was introduced in cups 2.3.
Only handle it when we build against a new enough cups.

Fixes: #2968

modules/printbackends/gtkprintbackendcups.c

@@ -647,7 +647,9 @@ add_cups_options (const char *key,
                           custom_value = TRUE;
                           break;
 
+#if CUPS_VERSION_MAJOR == 2 && CUPS_VERSION_MINOR >= 3
                         case PPD_CUSTOM_UNKNOWN:
+#endif
                         default :
                           custom_value = FALSE;
                         }