libreswan: fix compilation with Linux 4.14

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>

net/libreswan/Makefile

@@ -81,6 +81,10 @@ MAKE_FLAGS+= \
     FINALRUNDIR="/var/run/pluto" \
     ARCH="$(LINUX_KARCH)" \
 
+ifdef CONFIG_LINUX_4_14
+	MAKE_FLAGS+= USE_XFRM_INTERFACE_IFLA_HEADER=true
+endif
+
 define Build/Prepare
 	$(call Build/Prepare/Default)
 	$(SED) 's,include $$$$(top_srcdir)/mk/manpages.mk,,g' \