According to a comment in programs/pluto/kernel_xfrm_interface.c:177:
* IFLA_XFRM_IF_ID was added to mainline kernel 4.19 linux/if_link.h
with older kernel headers 'make USE_XFRM_INTERFACE_IFLA_HEADER=true'
Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>