fix build with libgsf
Authored by: David Tardon 2017-12-14

--- a/src/lib/OutputFileHelper.cxx
+++ b/src/lib/OutputFileHelper.cxx
@@ -57,7 +57,7 @@
 OutputFileHelperImpl::~OutputFileHelperImpl()
 {
 #ifdef USE_GSF_OUTPUT
-	if (mpOutfile && !gsf_output_close((GsfOutput *) mpOutfile.get()))
+	if (mpOutfile && !gsf_output_close((GsfOutput *) mpOutfile))
 		fprintf(stderr, "ERROR : Couldn't close outfile\n");
 
 	if (mpOutfile)