gvc-channel-bar: Fix [Wbad-function-cast] warning
Fixes the warning:
../src/settings/gvc-channel-bar.c: In function 'gvc_channel_bar_set_is_amplified':
../src/settings/gvc-channel-bar.c:320:35: warning: cast from function call of type 'gdouble' {aka 'double'} to non-matching type 'int' [-Wbad-function-cast]
320 | if ((int)self->base_volume == (int)ADJUSTMENT_MAX_NORMAL) {
| ^