dmaengine: idxd: Change license on idxd.h to LGPL

This file was given GPL-2.0 license. But LGPL-2.1 makes more sense
as it needs to be used by libraries outside of the kernel source tree.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>