[PATCH] Makefile: add uapi level to dkms-install target

[Jason A. Donenfeld]

Hey Chet,

Thanks for the patch. Indeed I hadn't finished that aspect of the
netlink stuff, so I'm glad you caught that before I made a snapshot.
I'll likely rebase this into the big netlink commit.

@dkg -- you were wondering what the purpose is. This is for the new
uapi/ directory, which needs to be accounted for in dkms, since
uapi/wireguard.h has replaced uapi.h during the recent Netlink
rewrite.

Jason