dhcpcd: fix wrong macro name

Fixes:
http://autobuild.buildroot.net/results/788/788e7103c7c7c0199f221f42be6c22cc9eac3e0b/
http://autobuild.buildroot.net/results/673/673d4a0a334c4694cc4dd05ef99069f681f98f0a/
http://autobuild.buildroot.net/results/4bb/4bbf9e785eb9cb0f916f0a2bdaf7a6a5f639f7ea/



and many more.

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