Mark a very stupid sign bug with a preprocessor warning.

I'll fix that soon.  I want to take the time and see how
much the behavior changes.

[[Imported from SVN: r6739]]

dune/gfe/rotation.hh

@@ -244,6 +244,7 @@ public:
 
             for (int m=0; m<3; m++) {
                 
+#warning There is a bug in the following line!
                 result[m][i] = (norm<1e10) 
                     /** \todo Isn't there a better way to implement this stably? */
                     ? 0.5 * (i==m)