debian: Fix cross build.

Cross-building openvswitch with debuild -aARCH (or equivalent) fails
because the target architecture is not getting passed to configure.
Thus binaries like ovs-appctl get built using the build host
architecture.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>