Supplement warning suppressions for gcc 11.

src/xt_exchanger_irecv_isend.c

src/xt_exchanger_irecv_isend_packed.c

src/xt_exchanger_irecv_send.c

src/xt_exchanger_mix_isend_irecv.c

src/xt_mpi.c

src/xt_request_msgs.c

src/xt_request_msgs_packed.c

src/xt_xmap_dist_dir.c

src/xt_xmap_dist_dir_intercomm.c

src/xt_xmap_intersection.c

src/xt_xmap_intersection_ext.c