package/mongodb: fix build with gcc 11

package/mongodb: fix build with gcc 11

Fix the following build failure with gcc 11:

In file included from src/mongo/db/query/plan_cache.h:36,
                 from src/mongo/db/catalog/collection_info_cache.h:33,
                 from src/mongo/db/catalog/collection.h:42,
                 from src/mongo/db/exec/requires_collection_stage.h:32,
                 from src/mongo/db/exec/delete.h:32,
                 from src/mongo/db/query/internal_plans.h:33,
                 from src/mongo/db/query/internal_plans.cpp:32:
src/mongo/db/exec/plan_stats.h:214:10: error: 'optional' in namespace 'std' does not name a template type
  214 |     std::optional<std::string> replanReason;
      |          ^~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/8c0875fa413923504515a83d8b679366418c2444

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