Correct way of freeing memory in NGSPICE class

Strings created with strdup() should be deleted using free(), not
delete.

eeschema/sim/ngspice.cpp

@@ -206,7 +206,7 @@ bool NGSPICE::LoadNetlist( const string& aNetlist )
     ngSpice_Circ( lines.data() );
 
     for( auto line : lines )
-        delete line;
+        free( line );
 
     return true;
 }