debian: Fix dependencies for openvswitch-datapath-dkms package.

debian: Fix dependencies for openvswitch-datapath-dkms package.

The OVS kernel module, like other kernel modules, does not need a working
userspace build environment, but the OVS "configure" script and makefiles
don't support a kernel-only build, so "configure" fails if libc6-dev is
not installed.

It would be better to change the "configure" script and makefiles to handle
this special case, but that was more work than I initially expected it to
be.

Bug #8606.
Reported-by: Edwin Chiu <echiu@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>