Add the missing mpl_image_compare mark to test_plot_dataframe_incols

[seisman]

Description of proposed changes

Fix the following error shown in the CI runs:

pygmt/tests/test_plot.py::test_plot_dataframe_incols
  /usr/share/miniconda3/envs/pygmt/lib/python3.8/site-packages/_pytest/python.py:199: PytestReturnNotNoneWarning: Expected None, but pygmt/tests/test_plot.py::test_plot_dataframe_incols returned <pygmt.figure.Figure object at 0x7fb2c5134520>, which will be an error in a future version of pytest.  Did you mean to use `assert` instead of `return`?
    warnings.warn(

Fixes #

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
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• Write detailed docstrings for all functions/methods.
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