selftests/powerpc: Include asm/cputable.h from utils.h

utils.h provides have_hwcap() and have_hwcap2() which check for a
feature bit. Those bits are defined in asm/cputable.h, so include it
in utils.h so users of utils.h don't have to do it manually.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-4-mpe@ellerman.id.au