ncs36510: fix sleep header private file

This private header should have unique name, that does not collide with
generic names like sleep.h

targets/TARGET_ONSEMI/TARGET_NCS36510/sleep_api.c

@@ -33,7 +33,8 @@
  */
 
 #if DEVICE_SLEEP
-#include "sleep.h"
+
+#include "fncs36510_sleep.h"
 #include "sleep_api.h"
 #include "cmsis_nvic.h"
 
@@ -47,4 +48,4 @@ void deepsleep()
     fncs36510_deepsleep();
 }
 
-#endif /* DEVICE_SLEEP */
+#endif /* DEVICE_SLEEP */