kvmtool: headers: update to Linux v4.6 release

Update our copy of the KVM header files to match the kernel's v4.6.0.
This fixes the ARM PMU support, where the feature identifier was
changed during the merge window due to a merge conflict.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>