Fix VS2013 build
Fix VS2013 build

Commit 9498b68 only dealt with VS2012 builds and broke VS2013 build. This patch
restores parity.

Signed-off-by: Paul Durrant <dba6cd059c0054f8a3c62ba902cd9d7238853ee4@citrix.com>

QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface,QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface,omeg/xeniface,omeg/xeniface,QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface,omeg/xeniface