arm64: Add kasan_hw_tags_enable() prototype to silence sparse

This function is only called from assembly, no need for a prototype
declaration in a header file. In addition, add #ifdef around the
function since it is only used when CONFIG_KASAN_HW_TAGS.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: kernel test robot <lkp@intel.com>