Fix headless build

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

include/plugincontext.hpp

@@ -20,12 +20,7 @@ struct Message;
 // Base DISTRHO classes
 
 #ifndef DISTRHO_DETAILS_HPP_INCLUDED
-
 namespace CardinalDISTRHO {
-
-class Plugin;
-class UI;
-
 struct MidiEvent {
     static const uint32_t kDataSize = 4;
     uint32_t frame;
@@ -33,9 +28,19 @@ struct MidiEvent {
     uint8_t data[kDataSize];
     const uint8_t* dataExt;
 };
+}
+#endif
 
+#ifndef DISTRHO_PLUGIN_HPP_INCLUDED
+namespace CardinalDISTRHO {
+class Plugin;
 }
+#endif
 
+#ifndef DISTRHO_UI_HPP_INCLUDED
+namespace CardinalDISTRHO {
+class UI;
+}
 #endif
 
 // --------------------------------------------------------------------------------------------------------------------