powerpc/vdso: Cleanup vdso.h

Rename the guard define to _ASM_POWERPC_VDSO_H

And remove useless #ifdef __KERNEL__

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9902590d410cd1c2afa48b83b277faf0711f07b2.1601197618.git.christophe.leroy@csgroup.eu