Only generate “override” in header files for C++ generator.

Author: tritao

src/Generator/Generators/C/CCodeGenerator.cs

@@ -363,7 +363,7 @@ public override void GenerateMethodSpecifier(Method method, Class @class)
 
             Write(")");
 
-            if (method.IsOverride)
+            if (method.IsOverride && isHeaderFile)
                 Write(" override");
         }