gvc: squash -Wconversion warnings in 'gvevent_button_press'

gvc: squash -Wconversion warnings in 'gvevent_button_press'

Squashes the following compiler warnings. We know these are false positives due
to the switch cases in which this code appears.

  gvevent.c:373:23: warning: conversion from ‘int’ to ‘unsigned char’ may change
    value [-Wconversion]
    373 |         job->button = button;
        |                       ^~~~~~
  gvevent.c:378:23: warning: conversion from ‘int’ to ‘unsigned char’ may change
    value [-Wconversion]
    378 |         job->button = button;
        |                       ^~~~~~
  gvevent.c:384:23: warning: conversion from ‘int’ to ‘unsigned char’ may change
    value [-Wconversion]
    384 |         job->button = button;
        |                       ^~~~~~