ovl: remove unused forward declaration

The ovl_get_verity_xattr() function was never added, only its declaration.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Fixes: 184996e9 ("ovl: Validate verity xattr when resolving lowerdata")
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>