Use the result of converting to a unix path.

The commit 18a3195a ('Keep track of INCLUDE_DIRECTORIES as a vector
of structs.', 2012-11-19) moved the handling of includes from
cmGeneratorTarget to cmTarget, but in the process introduced this
bug.

Source/cmTarget.cxx

@@ -2797,10 +2797,10 @@ std::vector<std::string> cmTarget::GetIncludeDirectories(const char *config)
 
       if(uniqueIncludes.insert(inc).second)
         {
-        includes.push_back(*li);
+        includes.push_back(inc);
         if (debugIncludes)
           {
-          usedIncludes += " * " + *li + "\n";
+          usedIncludes += " * " + inc + "\n";
           }
         }
       }