The static mouse_report definition did not initialize the d_scroll_x
field. The zmk_hid_mouse_report_body structure contains five fields
(buttons, d_x, d_y, d_scroll_y, d_scroll_x), but d_scroll_x was left
uninitialized.
Initialize d_scroll_x to 0 to ensure all fields in the mouse report
are properly initialized and avoid potential undefined behavior.
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