dhcpcd: blacklist Sourcery PowerPC toolchains

The linux/socket.h header in these toolchains is broken as described in commit
f5866484 (libnftnl: exclude for CS PPC toolchains).

Fixes:
http://autobuild.buildroot.net/results/d92/d92e7f2af0e64bc40f7b5c9ea081471de98bf8e8/
http://autobuild.buildroot.net/results/e97/e97e980a2f1e0ff4138a6dc03d86ad3ff0973e4a/
http://autobuild.buildroot.net/results/ca3/ca32c4de8b645c96cedb80b6270a792e9601ad84/



and some more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>