Make hv_irq_mask() and hv_irq_unmask() static as they are only used in
pci-hyperv.c

This fixes a sparse warning.
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NK. Y. Srinivasan <kys@microsoft.com>