PCI: Restore PCI_MSIX_FLAGS_BIRMASK definition

09a2c73d ("PCI: Remove unused PCI_MSIX_FLAGS_BIRMASK definition")
removed PCI_MSIX_FLAGS_BIRMASK from an exported header because it was
unused in the kernel.  But that breaks user programs that were using it
(QEMU in particular).

Restore the PCI_MSIX_FLAGS_BIRMASK definition.

[bhelgaas: changelog]
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.13+