vmw_balloon: update maintainers list

Philip Moltman is no longer a maintainer of the VMware balloon. Setting
Nadav Amit as one instead.

Reviewed-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>