iwlwifi: remove linux/utsrelease.h dependency

Commit 250cce26 uses UTS_RELEASE
as the the in-tree iwlwifi driver version. However the inclusion
of generated/utsrelease.h makes it a unpleasant behaviour to
recompile the driver everytime when utsrelease.h is updated. In
fact, the driver module is already built with the UTS_RELEASE
information via vermagic of modinfo. Mark the in-tree driver
with the version string "in-tree" to distinguish with those old
out-of-tree drivers.

Reported-by: David Miller <davem@davemloft.net>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>