package/iptraf-ng: add dependency on Linux headers >= 3.14

package/iptraf-ng: add dependency on Linux headers >= 3.14

iptraf-ng uses a number of definitions (like PACKET_KERNEL and
PACKET_USERSPACE) provided by the Linux kernel headers, which were
introduced in Linux 3.14 [1], so we need to add a dependency on
BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_14.

[1] https://elixir.bootlin.com/linux/v3.14-rc1/source/include/uapi/linux/if_packet.h

Fixes:

  http://autobuild.buildroot.net/results/480b00b4b264cfe1da533c762595d21b72fa8211/

Signed-off-by: Jugurtha BELKALEM <jugurtha.belkalem@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>