Fixed linux opengl compile error.

modules/juce_opengl/native/juce_OpenGL_linux.h

@@ -106,7 +106,7 @@ public:
     void initialiseOnRenderThread()
     {
         ScopedXLock xlock;
-        renderContext = glXCreateContext (display, bestVisual, contextToShareWith, GL_TRUE);
+        renderContext = glXCreateContext (display, bestVisual, (GLXContext) contextToShareWith, GL_TRUE);
         makeActive();
     }