Add DISTRHO_API macro, unused for now

Signed-off-by: falkTX <falktx@falktx.com>

dgl/Application.hpp

@@ -33,7 +33,7 @@ START_NAMESPACE_DGL
 
    Unless stated otherwise, functions within this class are not thread-safe.
  */
-class Application
+class DISTRHO_API Application
 {
 public:
    /**

dgl/Window.hpp

@@ -52,7 +52,7 @@ class TopLevelWidget;
 
    ...
  */
-class Window
+class DISTRHO_API Window
 {
    struct PrivateData;
 

distrho/src/DistrhoDefines.h

@@ -27,10 +27,12 @@
 
 /* Check OS */
 #if defined(WIN32) || defined(_WIN32) || defined(__WIN32__)
+# define DISTRHO_API
 # define DISTRHO_PLUGIN_EXPORT extern "C" __declspec (dllexport)
 # define DISTRHO_OS_WINDOWS 1
 # define DISTRHO_DLL_EXTENSION "dll"
 #else
+# define DISTRHO_API
 # define DISTRHO_PLUGIN_EXPORT extern "C" __attribute__ ((visibility("default")))
 # if defined(__APPLE__)
 #  define DISTRHO_OS_MAC 1