envy24control: fix GtkSpinButton runtime warnings

envy24control: fix GtkSpinButton runtime warnings

Change the parameters to gtk_adjustment_new to get rid of this runtime
warning from GTK:

GtkSpinButton: setting an adjustment with non-zero page size is
deprecated

The change is also important when envy24control is ported to GTK 3, as
the spin buttons will stop working if it's not done.

Closes: https://github.com/alsa-project/alsa-tools/pull/31
Signed-off-by: Andreas Persson <andreasp56@outlook.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>