fix(rqt_diagnostic_graph_monitor): add missing dependency

[isamu-takagi]

Description

Fix missing dependency of rqt_diagnostic_graph_monitor package.

Related links

None

Tests performed

1. Build with --packages-up-to rqt_diagnostic_graph_monitor option
2. Run rqt_diagnostic_graph_monitor

Notes for reviewers

None

Interface changes

None

Effects on system behavior

None

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[TetsuKawa]

LGTM!