GCC 4.6 compatibility of Extension class.

It doesn't like const members, as std::vector doesn't have proper move
semantic yet.

src/Context.h

@@ -74,10 +74,12 @@ class MAGNUM_EXPORT Extension {
         inline constexpr const char* string() const { return _string; }
 
     private:
-        const size_t _index;
-        const Version _requiredVersion;
-        const Version _coreVersion;
-        const char* const _string;
+        /* GCC 4.6 doesn't like const members, as std::vector doesn't have
+           proper move semantic yet */
+        size_t _index;
+        Version _requiredVersion;
+        Version _coreVersion;
+        const char* _string;
 
         inline constexpr Extension(size_t index, Version requiredVersion, Version coreVersion, const char* string): _index(index), _requiredVersion(requiredVersion), _coreVersion(coreVersion), _string(string) {}
 };