memblock: Remove #ifdef __KERNEL__ from memblock.h

memblock.h is not a uAPI header, so __KERNEL__ guard can be deleted

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20220111102847.673746-1-karolinadrobnik@gmail.com