button: add udevice forward declaration

After 401d1c4f ("common: Drop asm/global_data.h from common header")
build fails with :

drivers/button/button-uclass.c:13:5: error: conflicting types for 'button_get_by_label'
int button_get_by_label(const char *label, struct udevice **devp)
    ^~~~~~~~~~~~~~~~~~~

Adding struct udevice forward declaration in button.h solves the build error.

Fixes: 401d1c4f ("common: Drop asm/global_data.h from common header")
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>