alpm: Fix function declaration isn’t a prototype error

../backends/alpm/pk-alpm-transaction.c:364:1: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
  364 | pk_alpm_transaction_output_end ()
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

backends/alpm/pk-alpm-transaction.c

@@ -361,7 +361,7 @@ pk_alpm_transaction_conv_cb (void *ctx, alpm_question_t *question)
 }
 
 static void
-pk_alpm_transaction_output_end ()
+pk_alpm_transaction_output_end (void)
 {
  tpkg = NULL;