Fixed compilation of EglContext.

Introduced in 59ca55abe0, we need 'None'
after all.

src/Contexts/EglContext.cpp

@@ -15,6 +15,8 @@
 
 #include "EglContext.h"
 
+#define None 0L // redef Xlib nonsense
+
 using namespace std;
 
 namespace Magnum { namespace Contexts {