dm: Protect device_unbind() with CONFIG_DM_DEVICE_REMOVE

Since device_unbind() is also defined in device-remove.c,
which is compiled in only in case CONFIG_DM_DEVICE_REMOVE
is defined, protect the device_unbind() prototype with the
same CONFIG_DM_DEVICE_REMOVE check.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>