package/nginx-naxsi: fix build without x_forwarded_for

package/nginx-naxsi: fix build without x_forwarded_for

Fix the following build failure without x_forwarded_for raised since
bump to version 1.3 in commit 3335c8d8685119ebb5b2a9a6e575f3a5337c4d90:

/home/buildroot/autobuild/instance-3/output-1/build/nginx-naxsi-1.3/naxsi_src/naxsi_runtime.c: In function 'ngx_http_naxsi_data_parse':
/home/buildroot/autobuild/instance-3/output-1/build/nginx-naxsi-1.3/naxsi_src/naxsi_runtime.c:2846:20: error: 'ngx_http_headers_in_t' has no member named 'x_forwarded_for'
   if (r->headers_in.x_forwarded_for.nelts >= 1) {
                    ^

Fixes:
 - http://autobuild.buildroot.org/results/cdbc1536f6b5de3d4c836efa2f0dcaf0cdbb1462

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>