netcat-openbsd: depends on linux-headers >= 3.12

This debian fork of netcat includes the kernel UAPI inet6 header. This
header conflicts with the existing libc inet header and causes
redefinition of structures when they are both included.

The compiler raises the following error:

	/home/buildroot/build/instance-0/output/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/linux/in6.h:30:8: error: redefinition of 'struct in6_addr' struct in6_addr {

>From version 3.12 of linux, the kernel UAPI header has mechanisms in
libc-compat.h to prevent such situations (see __UAPI_DEF_SOCKADDR_IN6
macros).

Unfortunately the build is broken when toolchains do not satisfy at least
the linux headers condition (>= 3.12).

The package now depends on the toolchain headers at least 3.12.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
[Thomas: update Config.in comment about the 3.12 headers dependency.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>