build: disable format-zero-length warning

build: disable format-zero-length warning

format-zero-length warns against printf-style calls with
zero-length format string. vfs_full_audit module has such
calls, and up until now there was no warning against it because
the do_log in vfs_full_audit is not recognized as printf-style
function. In a following commit the do_log will be converted to
a printf-style function, hence the need to disable this warning.

(an alternative would be to disable only for vfs_full_audit, but that
would complicate things needlessly).

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>