package/dhcpcd: create dhcpcd user

privsep is supported since version 9.0.0 and
https://github.com/rsmarples/dhcpcd/commit/d5786118da1bad4c247631cae86344f1b249a8cb
It is enabled by default since
https://github.com/rsmarples/dhcpcd/commit/3a4c2e5604d72151b06ed365aa71493740a3ad75

So use --privsepuser to avoid that the detection mechanism finds a wrong
value from host and create it on the target

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=13416



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