soundwire: include mod_devicetable.h to avoid compiling warnings

When integrating SoundWire, kbuild throws this warning with randconfig:

>> include/linux/soundwire/sdw.h:571:17: warning: 'struct
   sdw_device_id' declared inside parameter list will not be visible
   outside of this definition or declaration

       const struct sdw_device_id *id);
                    ^~~~~~~~~~~~~

Fix by adding the relevant include

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Bard liao <yung-chuan.liao@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190806005522.22642-8-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

include/linux/soundwire/sdw.h

@@ -4,6 +4,8 @@
 #ifndef __SOUNDWIRE_H
 #define __SOUNDWIRE_H
 
+#include <linux/mod_devicetable.h>
+
 struct sdw_bus;
 struct sdw_slave;