Gui: fix 'Minimum fractional inch' preference visibility

Default Unit Schema is not yet changed in onUnitSystemIndexChanged
callback handler, so index needs to be used to request currently
selected one.

Fixes: 1155f0d752 ("Base: simplify UnitsSchemas management")

src/Base/UnitsSchema.h

@@ -36,7 +36,7 @@ class Quantity;
 /**
  * An individual schema object
  */
-class UnitsSchema
+class BaseExport UnitsSchema
 {
 public:
     explicit UnitsSchema(UnitsSchemaSpec spec);