Copter: fix build for lack of AC_TERRAIN define

ArduCopter/Parameters.cpp

@@ -1079,7 +1079,7 @@ const AP_Param::GroupInfo ParametersG2::var_info[] = {
     AP_GROUPINFO("GUID_TIMEOUT", 46, ParametersG2, guided_timeout, 3.0),
 #endif
 
-#if AP_TERRAIN_AVAILABLE && AC_TERRAIN
+#if AP_TERRAIN_AVAILABLE
     // @Param: TERRAIN_MARGIN
     // @DisplayName: Terrain following altitude margin
     // @Description: Terrain following altitude margin.  Vehicle will stop if distance from target altitude is larger than this margin (in meters)

ArduCopter/Parameters.h

@@ -653,7 +653,7 @@ public:
     AP_Float guided_timeout;
 #endif
 
-#if AP_TERRAIN_AVAILABLE && AC_TERRAIN
+#if AP_TERRAIN_AVAILABLE
     AP_Float terrain_margin;
 #endif
 };

ArduCopter/terrain.cpp

@@ -30,7 +30,7 @@ void Copter::terrain_logging()
 // return terrain following altitude margin.  Vehicle will stop if distance from target altitude is larger than this margin (in meters)
 float Copter::get_terrain_margin() const
 {
-#if AP_TERRAIN_AVAILABLE && AC_TERRAIN
+#if AP_TERRAIN_AVAILABLE
     return MAX(g2.terrain_margin.get(), 0.1);
 #else
     return 10;