AP_Landing: change AC_FENCE to AP_FENCE_ENABLED

libraries/AP_Landing/AP_Landing.cpp

@@ -267,7 +267,7 @@ bool AP_Landing::verify_abort_landing(const Location &prev_WP_loc, Location &nex
              mission.resume();
          }
          // else we're in AUTO with a stopped mission and handle_auto_mode() will set RTL
-#if AC_FENCE
+#if AP_FENCE_ENABLED
         AC_Fence *fence = AP::fence();
         if (fence) {
             fence->auto_enable_fence_after_takeoff();