Some header inclusion clean-ups.

o Include what is directly used, nothing more.
 o Don't rely on transitive inclusion.
 o Order inclusions as: 1) own, 2) Qt, 3) others.
 o Order inclusions alphabetically.
 o Use forward declarations where possible.
 o Blank line between include guard and #include directives.

Tested on Qt 4.8.6 and Qt 5.4.1.

This incidentally fixes "invalid use of incomplete type ‘class
QStringList’" which I got in pack_p.cpp with Qt 4.8.6.

src/private/unpack_p.cpp

@@ -1,6 +1,9 @@
 #include "unpack_p.h"
 #include "sysdep.h"
+
+#include <QByteArray>
 #include <QDebug>
+#include <QMap>
 
 MsgPackPrivate::type_parser_f MsgPackPrivate::unpackers[32] = {
     unpack_nil,