Use same version in <linux/version.h> and uname()

We had a different Linux version compiled into uname() (3.7.0) than
we had compiled into other code (via <linux/version.h>).

This patch makes them both pretend to be 3.7.0 - arbitrarily chosen
as the current Linux version at the time OSv was created. The new code
verifies with a static assertion that both files contain the same version,
so if they diverge, uname.c will fail compilation.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>