ADB: Add missing #include <linux/platform_device.h>

Commit c9f6d3d5c6d4f4cd3a53549a69c92951180e2a76 ("[POWERPC] adb: Replace
sleep notifier with platform driver suspend/resume hooks") introduced
compile errors on m68k because <linux/platform_device.h> is not
explicitly included.  On powerpc, it's pulled in through <asm/prom.h>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>