Add QT_BEGIN_HEADER & QT_END_HEADER macros (#6865)

The macros `QT_BEGIN_HEADER` and `QT_END_HEADER` was removed from Qt in
2012.

Empty since 2012: 

qt/qtbase@ba3dc5f#diff-25c4cd4ef98b78f1b822c473dbbb29beb03092d0629b2c82982260ade1aa8521L143-L148

Removed completely in Qt6.0: 

qt/qtbase@2584998#diff-25c4cd4ef98b78f1b822c473dbbb29beb03092d0629b2c82982260ade1aa8521L213-L216

Author: bjornblissing

cfg/qt.cfg

@@ -5373,6 +5373,8 @@
   <define name="QT_END_NAMESPACE" value=""/>
   <define name="QT_BEGIN_MOC_NAMESPACE" value=""/>
   <define name="QT_END_MOC_NAMESPACE" value=""/>
+  <define name="QT_BEGIN_HEADER" value=""/>
+  <define name="QT_END_HEADER" value=""/>
   <define name="QT_TR_NOOP(x)" value="x"/>
   <define name="QT_TR_NOOP_UTF8(x)" value="x"/>
   <define name="QT_TRANSLATE_NOOP(scope, x)" value="x"/>