plugin, down-root: Fix compiler warnings

Removed a few compiler warnings:
  down-root.c:164:4: warning: implicit declaration of function 'warn'
[-Wimplicit-function-declaration]
  down-root.c:239:5: warning: implicit declaration of function 'err'
[-Wimplicit-function-declaration]
  down-root.c:461:7: warning: unused variable 'i' [-Wunused-variable]
  down-root.c:460:15: warning: unused variable 'p' [-Wunused-variable]

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: 1418074541-24987-1-git-send-email-openvpn.list@topphemmelig.net
URL: http://article.gmane.org/gmane.network.openvpn.devel/9327
(cherry picked from commit 7dd51f6f50b17ab91cbb724e2d5e96657fab834a)