wayland: use correct enum type for tablet pad

Fixes a compile warning. The two enums are identical, so no functional change intended.

For reference the two generated enums look like this:

enum zwp_tablet_tool_v2_button_state {
	ZWP_TABLET_TOOL_V2_BUTTON_STATE_RELEASED = 0,
	ZWP_TABLET_TOOL_V2_BUTTON_STATE_PRESSED = 1,
};

enum zwp_tablet_pad_v2_button_state {
	ZWP_TABLET_PAD_V2_BUTTON_STATE_RELEASED = 0,
	ZWP_TABLET_PAD_V2_BUTTON_STATE_PRESSED = 1,
};