package/libseccomp: needs headers >= 3.12

EM_AARCH64 was added to linux kernel with version 3.12:
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/log/include/uapi/linux/elf-em.h?h=linux-3.12.y

Fixes
http://autobuild.buildroot.net/results/4cd/4cd6aaccde9fb0f80e49133b477f330b601f4a63/
http://autobuild.buildroot.net/results/bd1/bd105e821de583e582199aae31fab13ef17e67a0/
http://autobuild.buildroot.net/results/092/092143df2b91d50850067274da531c713f2ccb72/
http://autobuild.buildroot.net/results/814/8149cf0c3ef3ef8d91b5891591a71691dc3d7427/



[Thomas: fix Config.in comment.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>