refs: add missing function declaration

The function `git_reference__is_note` is not declared anywhere. Let's
add the declaration to avoid having non-static functions without
declaration.

src/refs.h

@@ -90,6 +90,7 @@ int git_reference__is_valid_name(const char *refname, unsigned int flags);
 int git_reference__is_branch(const char *ref_name);
 int git_reference__is_remote(const char *ref_name);
 int git_reference__is_tag(const char *ref_name);
+int git_reference__is_note(const char *ref_name);
 const char *git_reference__shorthand(const char *name);
 
 /**