Added static_cast<void> to avoid warnings about unused variables

opm/core/io/eclipse/EclipseGridParser.cpp

@@ -1106,7 +1106,9 @@ void EclipseGridParser::saveEGRID_INIT( const std::string& output_dir , const st
 
 void EclipseGridParser::saveEGRID( const std::string & filename, int num_cells , const int * global_cell) const
 {
-    static_cast<void>(filename); // Suppress "unused variable" warning.
+    static_cast<void>(filename);    // Suppress "unused variable" warning.
+    static_cast<void>(num_cells);   // Suppress "unused variable" warning.
+    static_cast<void>(global_cell); // Suppress "unused variable" warning.
     THROW("Cannot write EGRID format without ERT library support. Reconfigure opm-core with ERT support and recompile.");
 }