kernel_patch_verify: allow for users to native builds

Obviously a "duh" moment.. no support for "not supporting"
CROSS_COMPILE is not really a good thing to do..

Fixes: https://github.com/nmenon/kernel_patch_verify/issues/2
Reported-by: Greg Kroah-Hartman <greg@kroah.com>
Signed-off-by: Nishanth Menon <nm@ti.com>