dhcpcd: needs kernel headers >= 3.1

dhcpcd: needs kernel headers >= 3.1

commit dfbfa6ef (dhcpcd: blacklist Sourcery PowerPC toolchains) blacklisted
specific toolchains, but the issue is actually that the kernel headers
before 3.1 didn't define sa_family_t, so instead require 3.1+ headers.

The specific kernel change fixing the headers is:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6602a4baf4d1a73cc

The only reverse dependency of dhcpcd (network-manager) already needs 3.7+,
so this dependency doesn't need to be propagated.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>