hw/gpio/aspeed_gpio.c: Don't directly include assert.h

Remove a direct include of assert.h -- this is already
provided by qemu/osdep.h, and it breaks our rule that the
first include must always be osdep.h.

In particular we must get the assert() macro via osdep.h
to avoid compile failures on mingw (see the comment in
osdep.h where we redefine assert() for that platform).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20200403124712.24826-1-peter.maydell@linaro.org