Prevent discarding const modifier

Addressing:
/home/jcerny/openscap/src/XCCDF/xccdf_session.c: In function ‘_build_xccdf_result_source’:
/home/jcerny/openscap/src/XCCDF/xccdf_session.c:1286:30: warning: passing argument 1 of ‘xccdf_benchmark_add_result’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 1286 |   xccdf_benchmark_add_result(benchmark, cloned_result);
      |                              ^~~~~~~~~
In file included from /home/jcerny/openscap/src/XCCDF_POLICY/public/xccdf_policy.h:34,
                 from /home/jcerny/openscap/src/OVAL/public/oval_agent_xccdf_api.h:41,
                 from /home/jcerny/openscap/src/XCCDF/xccdf_session.c:38:
/home/jcerny/openscap/src/XCCDF/public/xccdf_benchmark.h:3141:67: note: expected ‘struct xccdf_benchmark *’ but argument is of type ‘const struct xccdf_benchmark *’
 3141 | OSCAP_API bool xccdf_benchmark_add_result(struct xccdf_benchmark *bench, struct xccdf_result *result);
      |                                           ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/home/jcerny/openscap/src/XCCDF/xccdf_session.c:1287:64: warning: passing argument 1 of ‘xccdf_benchmark_export_source’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 1287 |   session->xccdf.result_source = xccdf_benchmark_export_source(benchmark, session->export.xccdf_file);
      |                                                                ^~~~~~~~~
In file included from /home/jcerny/openscap/src/XCCDF_POLICY/public/xccdf_policy.h:34,
                 from /home/jcerny/openscap/src/OVAL/public/oval_agent_xccdf_api.h:41,
                 from /home/jcerny/openscap/src/XCCDF/xccdf_session.c:38:
/home/jcerny/openscap/src/XCCDF/public/xccdf_benchmark.h:722:32: note: expected ‘struct xccdf_benchmark *’ but argument is of type ‘const struct xccdf_benchmark *’
  722 | OSCAP_API struct oscap_source *xccdf_benchmark_export_source(struct xccdf_benchmark *benchmark, const char *filename);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Author: jan-cerny

src/XCCDF/xccdf_session.c

@@ -1275,7 +1275,7 @@ static int _build_xccdf_result_source(struct xccdf_session *session)
 
 	/* Build oscap_source of XCCDF TestResult only when needed */
 	if (session->export.xccdf_file != NULL || session->export.report_file != NULL || session->export.arf_file != NULL || session->export.xccdf_stig_viewer_file != NULL) {
-		const struct xccdf_benchmark *benchmark = xccdf_policy_model_get_benchmark(session->xccdf.policy_model);
+		struct xccdf_benchmark *benchmark = xccdf_policy_model_get_benchmark(session->xccdf.policy_model);
 
 		if (session->xccdf.result == NULL) {
 			// Attempt to export session before evaluation