RISC-V: kasan: Declare kasan_shallow_populate() static

Without this I get a missing prototype warning.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: e178d670

 ("riscv/kasan: add KASAN_VMALLOC support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>