arm64: cpufeature.h: add missing #include of kernel.h

cpufeature.h makes use of DECLARE_BITMAP, which in turn relies on the
BITS_TO_LONGS and DIV_ROUND_UP macros.

This patch includes kernel.h in cpufeature.h to prevent all users having
to do the same thing.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>