debian: Correct licensing information for user/kernel shared header files.

debian: Correct licensing information for user/kernel shared header files.

The intent is to license all shared user/kernel header files under both
GPLv2 and Apache v2.  The license statement here said GPLv3 instead of
GPLv2, so this commit fixes that problem.

Also, include/openvswitch used to be where all the shared user/kernel
header files were located, but this is no longer true, and now there is a
userspace-only header file also in include/openvswitch, so this commit now
lists all of the user/kernel header files explicitly.

Signed-off-by: Ben Pfaff <blp@nicira.com>