kws: fix aws demo implicit declaration

Include ml_interface to fix:
> error: implicit declaration of function 'ml_task_inference_start' is invalid in C99

Signed-off-by: Martin Kojtal <martin.kojtal@arm.com>

kws/source/aws_demo.c

@@ -19,6 +19,8 @@
 #include "ota_provision.h"
 #include "version/application_version.h"
 
+#include "ml_interface.h"
+
 #define IOT_LOGGING_QUEUE_SIZE 90
 
 extern void DEMO_RUNNER_RunDemos(void);