Fix FreeBSD build

On FreeBSD, I need to change `selectedFilter = fileDialog-
>selectedFilter();` to `selectedFilter = fileDialog-
>selectedNameFilter();`

I think it's a reliquat of Qt4 since Qt5 doesn't have this method
anymore.

Author: lbartoletti

src/gui/qgsguiutils.cpp

@@ -158,7 +158,7 @@ namespace QgsGuiUtils
       outputFileName = fileDialog->selectedFiles().first();
     }
 
-    selectedFilter = fileDialog->selectedFilter();
+    selectedFilter = fileDialog->selectedNameFilter();
     QgsDebugMsg( "Selected filter: " + selectedFilter );
     ext = filterMap.value( selectedFilter, QString() );