AR_WPNav: stop libraries including AP_Logger.h in .h files

AP_Logger.h is a nexus of includes; while this is being improved over
time, there's no reason for the library headers to include AP_Logger.h
as the logger itself is access by singleton and the structures are in
LogStructure.h

This necessitated moving The PID_Info structure out of AP_Logger's
namespace.  This cleans up a pretty nasty bit - that structure is
definitely not simply used for logging, but also used to pass pid
information around to controllers!

There are a lot of patches in here because AP_Logger.h, acting as a
nexus, was providing transitive header file inclusion in many (some
unlikely!) places.

libraries/AR_WPNav/AR_PivotTurn.cpp

@@ -13,6 +13,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <AP_AHRS/AP_AHRS.h>
 #include <AP_HAL/AP_HAL.h>
 #include <AP_Math/AP_Math.h>
 #include "AR_PivotTurn.h"

libraries/AR_WPNav/AR_WPNav.cpp

@@ -13,6 +13,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <AP_AHRS/AP_AHRS.h>
 #include <AP_Math/AP_Math.h>
 #include <AP_HAL/AP_HAL.h>
 #include "AR_WPNav.h"

libraries/AR_WPNav/AR_WPNav_OA.cpp

@@ -13,6 +13,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <AP_AHRS/AP_AHRS.h>
 #include <AP_Math/AP_Math.h>
 #include <AP_HAL/AP_HAL.h>
 #include "AR_WPNav_OA.h"