igb_uio: fix build on Linux < 3.16

igb_uio: fix build on Linux < 3.16

Starting from Linux v3.16 pci_is_bridge() is in linux/pci.h,
 in previous versions it is in drivers/pci/pci.h which is private header.

Fix build error when calling pci_is_bridge by not calling/supporting
pci_is_bridge with kernel versions before 3.16.

Fixes: 6f0841b7701b ("igb_uio: bind error if PCIe bridge")

Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>