selftests: memfd: add config fragment for fuse

The memfd test requires to insert the fuse module (CONFIG_FUSE_FS).

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>