shared: guard all the seal logic behind HAVE_MEMFD_CREATE
The initial version of os_ro_anonymous_file missed two guards around the seal logic which leads to a compilation error on older systems.
Also make the check for a read-only file symmetric in os_ro_anonymous_file_get_fd and os_ro_anonymous_file_put_fd.
Signed-off-by: Sebastian Wick sebastian@sebastianwick.net