Add mising const in Ogre

Add the required const to operator == as required and warned about
with C++20.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/third_party/ogre3d_preprocessor/OgreGLSLPreprocessor.h

@@ -168,7 +168,7 @@ namespace Ogre
             void SetValue (long iValue);
 
             /// Test two tokens for equality
-            bool operator == (const Token &iOther)
+            bool operator == (const Token &iOther) const
             {
                 if (iOther.Length != Length)
                     return false;