Adapt to cups-filters 2.x changes: No output format parameters

Most filter functions do not need to have their output data format
specified as individual parameter but now input and output data format
are defined in the common filter data for the filter chain.

Therefore supply NULL as parameter for cfFilterPDFToPDF() now.

ghostscript-printer-app.c

@@ -55,7 +55,7 @@ static pr_spooling_conversion_t ghostscript_convert_pdf_to_fx_pdf =
   {
     {
       cfFilterPDFToPDF,
-      "application/vnd.cups-pdf",
+      NULL,
       "pdftopdf"
     },
     {