When element references simple type it should still be considered to …

…be "string"

Addressing:
 # github.com/gocomply/scap/pkg/scap/models/oval_def
pkg/scap/models/oval_def/models.go:106:20: cannot refer to unexported name oval.string
pkg/scap/models/oval_def/models.go:106:20: undefined: oval.string

pkg/xsd/element.go

@@ -107,7 +107,7 @@ func (e *Element) ContainsText() bool {
 }
 
 func (e *Element) isPlainString() bool {
-	return e.SimpleType != nil || (e.Type == "" && e.Ref == "" && e.ComplexType == nil)
+	return e.SimpleType != nil || (e.Type == "" && e.Ref == "" && e.ComplexType == nil) || (e.typ != nil && e.typ.GoTypeName() == "string")
 }
 
 func (e *Element) isArray() bool {