headers: fix build error in arch/arm/plat-samsung/adc.c, add <linux/mod_devicetable.h>

arch/arm/plat-samsung/adc.c needs to #include <linux/mod_devicetable.h>
explicitly since that header file was removed from
<linux/platform_device.h> (it wasn't needed there).

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>