Fix for C++ header in public Objective-C header (#2156)

platform/darwin/app/ExampleCustomDrawableStyleLayer.mm

@@ -13,6 +13,10 @@
 #include <memory>
 #include <cmath>
 
+@interface MLNCustomDrawableStyleLayer (Internal)
+- (instancetype)initWithPendingLayer:(std::unique_ptr<mbgl::style::Layer>)pendingLayer;
+@end
+
 class ExampleCustomDrawableStyleLayerHost;
 
 @implementation ExampleCustomDrawableStyleLayer

platform/darwin/src/MLNCustomDrawableStyleLayer.h

@@ -6,15 +6,6 @@
 #import "MLNStyleLayer.h"
 #import "MLNGeometry.h"
 
-#ifdef __cplusplus
-#include <mbgl/style/layer.hpp>
-#endif
-
 MLN_EXPORT
 @interface MLNCustomDrawableStyleLayer : MLNStyleLayer
-
-#ifdef __cplusplus
-- (instancetype)initWithPendingLayer:(std::unique_ptr<mbgl::style::Layer>)pendingLayer;
-#endif
-
 @end

platform/darwin/src/MLNCustomDrawableStyleLayer.mm

@@ -12,6 +12,12 @@
 #include <memory>
 #include <cmath>
 
+#include <mbgl/style/layer.hpp>
+
+@interface MLNCustomDrawableStyleLayer (Internal)
+- (instancetype)initWithPendingLayer:(std::unique_ptr<mbgl::style::Layer>)pendingLayer;
+@end
+
 @implementation MLNCustomDrawableStyleLayer
 
 - (instancetype)initWithRawLayer:(mbgl::style::Layer *)rawLayer {