dhcpcd: disable on static library builds

dhcpcd uses dlopen(), so it is not compatible with pure static library
builds.

Fixes:

  http://autobuild.buildroot.net/results/512/51249e8d3487e17bb6a60a99dcbd461f7b591eac/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>