jtag: drivers: xlnx-pcie-xvc: fix build on Linux pre v4.10

jtag: drivers: xlnx-pcie-xvc: fix build on Linux pre v4.10

The macro PCI_CFG_SPACE_EXP_SIZE is exposed to userspace from
Linux kernel v4.10, with commit cc10385b6fde ("PCI: Move config
space size macros to pci_regs.h")
http://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc10385b6fde

Define the macro in the driver code, if not already defined.

Change-Id: I610219a2587eff2c142102b9f7830e3da882af78
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5435
Reviewed-by: Moritz Fischer <moritzf@google.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Tested-by: jenkins