ARROW-16296: [GLib] Add missing casts for GArrowRoundMode
ARROW-16296: [GLib] Add missing casts for GArrowRoundMode

This fixes the following warning:

    GLib-GObject-WARNING **: value "((GArrowRoundMode) -765521144)"
    of type 'GArrowRoundMode' is invalid or out of range for property
    'mode' of type 'GArrowRoundMode'

Closes #12971 from kou/glib-compute-round-options-missing-cast

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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