AP_DAL: avoid using struct Location

clang reports this could be a problem when compiling under some EABIs.  Remove it from most places as it is just noise, replace with class where we want to avoid including Location.h

libraries/AP_DAL/AP_DAL.h

@@ -190,7 +190,7 @@ public:
 
     // get the home location. This is const to prevent any changes to
     // home without telling AHRS about the change
-    const struct Location &get_home(void) const {
+    const class Location &get_home(void) const {
         return _home;
     }