ENH: Encapsulate expat header

Don't include the expat header in itkXMLFile.h, this improves
encapsulation and reduces potential problems mixing expat version.

Change-Id: Ida6dcb33d9407aa1bd47437d43f75c872abb7f78
Merge topic 'EcapsulateExpat'

47c44b64 ENH: Encapsulate expat header