PCI: Restore PCI_MSIX_FLAGS_BIRMASK definition

09a2c73ddf ("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+

include/uapi/linux/pci_regs.h

@@ -319,6 +319,7 @@
 #define PCI_MSIX_PBA		8	/* Pending Bit Array offset */
 #define  PCI_MSIX_PBA_BIR	0x00000007 /* BAR index */
 #define  PCI_MSIX_PBA_OFFSET	0xfffffff8 /* Offset into specified BAR */
+#define PCI_MSIX_FLAGS_BIRMASK	PCI_MSIX_PBA_BIR /* deprecated */
 #define PCI_CAP_MSIX_SIZEOF	12	/* size of MSIX registers */
 
 /* MSI-X Table entry format */