AP_Common: Semaphore is missing an include

Everyone who is already including common/Semaphore has extra includes to make this compile. This reduces that dependancy

libraries/AP_Common/Semaphore.h

@@ -27,6 +27,8 @@
   The WITH_SEMAPHORE() macro can be used with either type of semaphore
  */
 
+#include <AP_HAL/Semaphores.h>
+
 namespace AP_HAL {
 class Semaphore;
 }