machines: Dont lock linux-libc-headers-nativesdk preferred provider

machines: Dont lock linux-libc-headers-nativesdk preferred provider

linux-libc-headers-nativesdk is for native bits in an SDK
its not required to be locked in machine conf files since
nothing depends on machine per se here. If its a mattetr of
policy then this should be added to distro conf which
in this layer is fsl.conf

Signed-off-by: Khem Raj <raj.khem@gmail.com>