NFS client is already enabled (NFS_FS) and by default it enables clients
for version 2 and 3. Enable explicitly the version 4 client to utilize
the newer protocol.

The NFS client is especially useful for testing kernel in automated
environments (network boot with network file system).
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>