sst: fix compile error

Add several include files to fix the below compile error.

drivers/staging/intel_sst/intelmid.c: In function ‘snd_intelmad_sst_register’:
drivers/staging/intel_sst/intelmid.c:805:2: error: ‘sst_drv_ctx’ undeclared (first use in this function)
drivers/staging/intel_sst/intelmid.c:805:2: note: each undeclared identifier is reported only once for each function it appears in

Signed-off-by: Lu Guanqun <guanqun.lu@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/staging/intel_sst/intelmid.c

@@ -32,6 +32,7 @@
 #include <linux/platform_device.h>
 #include <linux/interrupt.h>
 #include <linux/sched.h>
+#include <linux/firmware.h>
 #include <sound/control.h>
 #include <asm/mrst.h>
 #include <sound/pcm.h>
@@ -40,6 +41,8 @@
 #include <sound/initval.h>
 #include "intel_sst.h"
 #include "intel_sst_ioctl.h"
+#include "intel_sst_fw_ipc.h"
+#include "intel_sst_common.h"
 #include "intelmid_snd_control.h"
 #include "intelmid.h"