Minor cleanup.

Fixes Clang warnings.

src/AbstractObject.h

@@ -42,8 +42,7 @@ namespace Implementation { struct DebugState; }
 @brief Base for all OpenGL objects
 */
 class MAGNUM_EXPORT AbstractObject {
-    friend class Context;
+    friend struct Implementation::DebugState;
-    friend class Implementation::DebugState;
 
     public:
         /**

src/DebugMessage.h

@@ -69,7 +69,7 @@ With default callback the messages will be printed on standard output:
         Hello from OpenGL command stream!
 */
 class MAGNUM_EXPORT DebugMessage {
-    friend class Implementation::DebugState;
+    friend struct Implementation::DebugState;
 
     public:
         /**