package/qemu: Compare host/target Linux version only during build phase

This patch fixes an error during source-check.

Thanks to Thomas for the hint:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/116183



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>