[WFLY-3103] fix PrimitiveListAttributeDefinition.Builder

take the copied AttributeDefinition's elementValidator to set the
builder's validator.

JIRA: https://issues.jboss.org/browse/WFLY-3103
Merge pull request #6028 from jmesnil/WFLY-3103_PrimitiveListAttributeDefinition.Builder_fix

[WFLY-3103] fix PrimitiveListAttributeDefinition.Builder