fb/exynos: include platform_device.h

fb/exynos: include platform_device.h

Patch dfe8de7 "kgdb: remove #include <linux/serial_8250.h> from kgdb.h"
removed an implicit inclusion of linux/platform_device.h
from the exynos framebuffer driver. This adds back the required
explicit header file inclusions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ajay Kumar <ajaykumar.rs@samsung.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>