COMP: On windows, fix unresolved external symbol "private: static class PythonQt * PythonQt::_self"

When classes were linking against PythonQt.lib, the linker was
complaining.

Moving the implementation of the static method "self()"  to
the cpp file solved the problem.

Author: jcfr

src/PythonQt.cpp

@@ -142,6 +142,8 @@ void PythonQt::cleanup()
   }
 }
 
+PythonQt* PythonQt::self() { return _self; }
+
 PythonQt::PythonQt(int flags, const QByteArray& pythonQtModuleName)
 {
   _p = new PythonQtPrivate;

src/PythonQt.h

@@ -158,7 +158,7 @@ class PYTHONQT_EXPORT PythonQt : public QObject {
   static void cleanup();
 
   //! get the singleton instance
-  static PythonQt* self() { return _self; }
+  static PythonQt* self();
 
   //@}