systemd: downgrade kernel headers minimum version to 3.7

The kernel headers minimum version for systemd was bumped to 3.8
(IFLA_GRE_FLOWINFO and IFLA_IPTUN_FLAGS definitions) and then to 3.10
(IFLA_VLAN_PROTOCOL definition). However, systemd since version v215
contains the following patch that defines IFLA_* constants if
needed:

  81577dc missing.h: add various network enums
  http://cgit.freedesktop.org/systemd/systemd/commit/?id=81577dc



As a consequence, this patch downgrades the kernel headers minimum
version to linux 3.7 which is required for BPF_XOR.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>