mgr/dashboard: Add # type ignore to fix ci failure

mgr/dashboard: Add # type ignore to fix ci failure

issue:
=====
The run_tox py is failing :
dashboard/module.py:34: note: In module imported here,
dashboard/__init__.py:60: note: ... from here:
dashboard/services/rgw_client.py: note: In member "_is_system_user" of class "RgwClient":
dashboard/services/rgw_client.py:483: error: Incompatible return value type (got "Literal[0, 1]", expected "bool")
Found 1 error in 1 file (checked 32 source files)
mypy: exit 1 (82.29 seconds) /home/jenkins-build/build/workspace/ceph-pull-requests/src/pybind/mgr> mypy --config-file=../../mypy.ini -m alerts -m balancer -m cephadm -m crash -m dashboard -m devicehealth -m diskprediction_local -m hello -m influx -m iostat -m localpool -m mds_autoscaler -m mgr_module -m mgr_util -m mirroring -m nfs -m orchestrator -m pg_autoscaler -m progress -m prometheus -m rbd_support -m rgw -m rook -m snap_schedule -m selftest -m stats -m status -m telegraf -m telemetry -m test_orchestrator -m volumes -m zabbix pid=235356
mypy: FAIL ✖ in 3 minutes 11.62 secondstake your time..you can postpone too

Fix:
===
Added "# type ignore" to ignore the type check to _is_system_user's
response

Signed-off-by: Afreen <afreen23.git@gmail.com>