qemu-native: Use inherit_defer for including native class

qemu-native: Use inherit_defer for including native class

This helps working with native qemu recipes with devtool, since devtool
inserts an internal class called devtool-source resulting in

ERROR: QA Issue: qemu-system-native: native/nativesdk class is not inherited last, this can result in unexpected behaviour. Classes i
nherited after native/nativesdk: devtool-source.bbclass [native-last]
ERROR: Fatal QA errors were found, failing task.
ERROR: Parsing halted due to errors, see error messages above
ERROR: Command execution failed:

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>