soc: sifive: l2 cache: Mark l2_get_priv_group as static

The kbuild test robot is firing a warning over a missing prototype.  The
function can just be static.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

drivers/soc/sifive/sifive_l2_cache.c

@@ -133,7 +133,7 @@ static const struct attribute_group priv_attr_group = {
 	.attrs = priv_attrs,
 };
 
-const struct attribute_group *l2_get_priv_group(struct cacheinfo *this_leaf)
+static const struct attribute_group *l2_get_priv_group(struct cacheinfo *this_leaf)
 {
 	/* We want to use private group for L2 cache only */
 	if (this_leaf->level == 2)