package/dhcpcd: fix build without fork

package/dhcpcd: fix build without fork

- Add a patch to fix build without fork in src/dhcpcd.c. This
  regression was introduced in upstream commit
  3063ebb6c8ac7c96196fa923cdd5f7c0384de23b, which was merged in dhcpcd
  9.0.0. Therefore, Buildroot is affected since we bumped from 8.0.3
  to 9.1.4 in commit 809f548e79c6c099f1fa3e3728d90842be7059a7, which
  was applied after 2020.05

- Disable privsep as it unconditionally uses fork (privsep has been
  enabled by default since version 9.0.0 and
  https://github.com/rsmarples/dhcpcd/commit/3a4c2e5604d72151b06ed365aa71493740a3ad75)

Fixes:

 - http://autobuild.buildroot.org/results/9fcc88abedcb8a02946f37837dcf4fff02f66c23

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>