OvmfPkg: Fix build failure with gcc44, gcc45

OvmfPkg/XenBusDxe/XenHypercall.h:19:31: error: redefinition of typedef 'XENBUS_DEVICE'
OvmfPkg/XenBusDxe/XenBusDxe.h:86:31: note: previous declaration of 'XENBUS_DEVICE' was here

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Scott Duplichan <scott@notabs.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@16408 6f19259b-4bc3-4df7-8a09-765794883524