samba4: requires NPTL

With OpenRISC there is a uClibc-ng based toolchain for an
architecture with MMU support, but with only Linuxthreads
and no NPTL. Samba4 uses pthread_mutexattr_setrobust which
is not implemented in uClibc-ng Linuxthreads.

Fixes:
  http://autobuild.buildroot.net/results/ec8f078046c92ae23dc973c88ac0fef136ad1535



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>