AC_Precland: fixed build under cygwin

the IRLock driver only builds on PX4 at the moment

libraries/AC_PrecLand/AC_PrecLand.cpp

@@ -75,9 +75,11 @@ void AC_PrecLand::init()
             _backend = new AC_PrecLand_Companion(*this, _backend_state);
             break;
         // IR Lock
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
         case PRECLAND_TYPE_IRLOCK:
             _backend = new AC_PrecLand_IRLock(*this, _backend_state);
             break;
+#endif
     }
 
     // init backend

libraries/AC_PrecLand/AC_PrecLand_IRLock.cpp

@@ -4,6 +4,9 @@
 
 extern const AP_HAL::HAL& hal;
 
+// this only builds for PX4 so far
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
+
 // Constructor
 AC_PrecLand_IRLock::AC_PrecLand_IRLock(const AC_PrecLand& frontend, AC_PrecLand::precland_state& state)
     : AC_PrecLand_Backend(frontend, state),
@@ -35,3 +38,5 @@ bool AC_PrecLand_IRLock::get_angle_to_target(float &x_angle_rad, float &y_angle_
 {
     return irlock.get_angle_to_target(x_angle_rad, y_angle_rad);
 }
+
+#endif // PX4

libraries/AC_PrecLand/AC_PrecLand_IRLock.h

@@ -7,6 +7,9 @@
 #include <AC_PrecLand/AC_PrecLand_Backend.h>
 #include <AP_IRLock/AP_IRLock.h>
 
+// this only builds for PX4 so far
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
+
 /*
  * AC_PrecLand_IRLock - implements precision landing using target vectors provided
  *                         by a companion computer (i.e. Odroid) communicating via MAVLink
@@ -36,4 +39,5 @@ private:
     AP_IRLock_PX4 irlock;
 
 };
+#endif
 #endif	// __AC_PRECLAND_IRLOCK_H__