arm64/hwcap: Include kernel-hwcap.h in list of generated files

When adding generation for the kernel internal constants for hwcaps the
generated file was not explicitly flagged as such in the build system,
causing it to be regenerated on each build. This wasn't obvious when the
series the change was included in was developed since it was all about
changes that trigger rebuilds anyway.

Fixes: abed23c3c44f ("arm64/hwcap: Generate the KERNEL_HWCAP_ definitions for the hwcaps")
Reported-by: Marek Vasut <marex@nabladev.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>