Remove dependency on us_ticker HAL apis for non USTICKER targets

- Surround mbed_us_ticker_api.c with if DEVICE_USTICKER
- get_us_ticker_data() returns NULL for non-usticker targets.

Author: Michael Schwarcz

hal/mbed_us_ticker_api.c

@@ -14,8 +14,12 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+#include <stddef.h>
 #include "hal/us_ticker_api.h"
 
+#if DEVICE_USTICKER
+
 static ticker_event_queue_t events = { 0 };
 
 static ticker_irq_handler_type irq_handler = ticker_irq_handler;
@@ -56,3 +60,12 @@ void us_ticker_irq_handler(void)
         irq_handler(&us_data);
     }
 }
+
+#else
+
+const ticker_data_t *get_us_ticker_data(void)
+{
+    return NULL;
+}
+
+#endif  // DEVICE_USTICKER