DKMS passes in an explicit variable for the kernel source directory, so we
should use that instead of `uname -r`.
CC: 659685@bugs.debian.org Reported-by: Thomas Goirand <thomas@goirand.fr> Tested-by: Simon Horman <horms@verge.net.au> Signed-off-by: Ben Pfaff <blp@nicira.com>