Fix the compilation with GCC 4.6's headers.

std::list is in #include <list>

/home/thiago/src/qt/creator/src/libs/cplusplus/pp-engine.cpp(110): error: qualified name is not allowed
      std::list<PPToken> tokens;
      ^

Change-Id: I02a08afe6a678d66d528ce5dfca08317735704e8
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/libs/cplusplus/pp-engine.cpp

@@ -60,6 +60,7 @@
 
 #include <QtDebug>
 #include <algorithm>
+#include <list>
 #include <QList>
 #include <QDate>
 #include <QTime>