wscript: use opt.PRIVATE_EXTENSION_DEFAULT('private-samba')

wscript: use opt.PRIVATE_EXTENSION_DEFAULT('private-samba')

The problem was that we used opt.PRIVATE_EXTENSION_DEFAULT('samba4') and
libndr as private will become libndr-samba4 and that already exists as
libndr-samba4 as we don't append the extension if it's already there.

So meant with --private-libraries=ALL we hit the following problem:

$ ./configure --private-libraries=ALL
$ make smbd/smbd
Waf: Leaving directory `/samba/bin/default'
Task dependency cycle in "run_after" constraints:
{task ...: cshlib dcerpc-samba4.empty.c.12.o,ndr_winbind_c.c.229.o -> libdcerpc-samba4.so}
make: *** [Makefile:131: smbd/smbd] Error 1

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15545

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Björn Jacke <bjacke@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>