arm-autonomy: fix building for xen-tools >=4.15

The 0001-vif-nat-fix-hostname.patch is merged in xen-tools
4.15, so fix the layer to apply the patch only for 4.14.

Issue-Id: SCM-2608
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Change-Id: I1306955ab0b1ee435a8da42e953e142270c4b2d3
Signed-off-by: Jon Mason <jon.mason@arm.com>