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>

arch/riscv/mm/kasan_init.c

@@ -155,7 +155,7 @@ static void __init kasan_populate(void *start, void *end)
 	memset(start, KASAN_SHADOW_INIT, end - start);
 }
 
-void __init kasan_shallow_populate(void *start, void *end)
+static void __init kasan_shallow_populate(void *start, void *end)
 {
 	unsigned long vaddr = (unsigned long)start & PAGE_MASK;
 	unsigned long vend = PAGE_ALIGN((unsigned long)end);