NULL: Fix compiler error when using NullMutexManager

The compiler error fixed (when trying to set the _mutexManager) is:
error: ‘MutexManager’ is an inaccessible base of ‘NullMutexManager’

1 files changed, 1 insertions, 1 deletions

diff --git a/backends/mutex/null/null-mutex.h b/backends/mutex/null/null-mutex.h
index fdb32b241c..7ae10cedb8 100644
--- a/backends/mutex/null/null-mutex.h
+++ b/backends/mutex/null/null-mutex.h
@@ -28,7 +28,7 @@
 /**
  * Null mutex manager
  */
-class NullMutexManager : MutexManager {
+class NullMutexManager : public MutexManager {
 public:
 	virtual OSystem::MutexRef createMutex() { return OSystem::MutexRef(); }
 	virtual void lockMutex(OSystem::MutexRef mutex) {}