package/openswan: disable -Werror

package/openswan: disable -Werror

Disable -Werror to avoid the following build failure with -DNDEBUG
raised since commit 5a8c50fe05afacc3cbe8e7347e238da9f242fab0

/srv/storage/autobuild/run/instance-2/output-1/build/openswan-3.0.0/programs/rsasigkey/rsasigkey.c:524:6: error: variable 'success' set but not used [-Werror=unused-but-set-variable]
  524 |  int success;
      |      ^~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/327a0f2b8f0c51bcbb3edb1c3671870d593e93b9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>