waf: Build nmbd with -Wno-error=stringop-overflow

waf: Build nmbd with -Wno-error=stringop-overflow

We use strlcpy() which has been added to glibc recently. This means we
also get fortification for strlcpy() now:

source3/nmbd/nmbd_browsesync.c: In function ‘find_domain_master_name_query_success’:
source3/nmbd/nmbd_browsesync.c:337:9: warning: ‘strlcpy’ writing 257 bytes into a
region of size 16 overflows the destination [-Wstringop-overflow=]
  337 |         strlcpy(userdata->data, work->work_group, size - sizeof(*userdata));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We allocate memory for the userdata struct + fstring. However the data
pointer we use only is 16 bytes. Also nowadays you would use offsetof()
for the allocation calculation, but it only works correctly on newer
compilers like gcc > 7. We could make use of it in future after CentOS 7
is gone.

As we don't want to touch nmbd anymore, just silence the warnings.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>