samba4: depend on host-nfs-utils

samba4: depend on host-nfs-utils

host-nfs-utils provides the host rpcgen utility. This fixes the build on
recent Fedora systems that removed rpcgen from the glibc package.

Fixes:
http://autobuild.buildroot.net/results/6f8/6f869b8c7c182dfe7f1a291b5952320504540ebf/
http://autobuild.buildroot.net/results/4c2/4c228dd6369e4fc11b798af6ab2abe06d7473b78/
http://autobuild.buildroot.net/results/489/489be441abd19fba2b0c39357e64a6adcf773b60/

Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>