PCI: hv: Make unnecessarily global IRQ masking functions static

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: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>