AP_NavEKF3: move definition of MAX_EKF_CORES

if you're not including the AP_NavEKF3 header then you don't get this definition and then you won't be able to compile the DAL.

libraries/AP_DAL/AP_DAL.cpp

@@ -8,6 +8,7 @@
 #include <AP_WheelEncoder/AP_WheelEncoder.h>
 #include <AP_Vehicle/AP_Vehicle_Type.h>
 #include <AP_NavEKF3/AP_NavEKF3_feature.h>
+#include <AP_NavEKF/AP_Nav_Common.h>
 
 #if APM_BUILD_TYPE(APM_BUILD_Replay)
 #include <AP_NavEKF2/AP_NavEKF2.h>

libraries/AP_NavEKF/AP_Nav_Common.h

@@ -19,6 +19,8 @@
 #include <stdint.h>
 #include <AP_Math/AP_Math.h>
 
+#define MAX_EKF_CORES     3 // maximum allowed EKF Cores to be instantiated
+
 // enumeration corresponding to buts within nav_filter_status union.
 // Only used for documentation purposes.
 enum class NavFilterStatusBit {

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -530,7 +530,6 @@ class NavEKF3 {
         float core_delta;             // the amount of D position change between cores when a change happened
     } pos_down_reset_data;
 
-#define MAX_EKF_CORES     3 // maximum allowed EKF Cores to be instantiated
 #define CORE_ERR_LIM      1 // -LIM to LIM relative error range for a core
 #define BETTER_THRESH   0.5 // a lane should have this much relative error difference to be considered for overriding a healthy primary core