Ap_Terrain: make AP::terrain return a pointer

this is needed for rover and OSD support

libraries/AP_Terrain/AP_Terrain.cpp

@@ -402,9 +402,9 @@ bool AP_Terrain::allocate(void)
 
 namespace AP {
 
-AP_Terrain &terrain()
+AP_Terrain *terrain()
 {
-    return *AP_Terrain::get_singleton();
+    return AP_Terrain::get_singleton();
 }
 
 };

libraries/AP_Terrain/AP_Terrain.h

@@ -440,7 +440,7 @@ private:
 };
 
 namespace AP {
-    AP_Terrain &terrain();
+    AP_Terrain *terrain();
 };
 
 #endif // AP_TERRAIN_AVAILABLE