iommu-common: fix x86_64 compiler warnings

Declare iommu_large_alloc as static. Remove extern definition  for
iommu_tbl_pool_init().

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>