permission: address coverity warning

The latest version of coverity has suggestions on how
to improve formance. Address one of these suggestions.

Signed-off-by: Michael Dawson <midawson@redhat.com>
PR-URL: #50215
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

src/permission/fs_permission.h

@@ -31,7 +31,7 @@ class FSPermission final : public PermissionBase {
 
       Node() : wildcard_child(nullptr), is_leaf(false) {}
 
-      Node* CreateChild(std::string prefix) {
+      Node* CreateChild(const std::string& prefix) {
         if (prefix.empty() && !is_leaf) {
           is_leaf = true;
           return this;