Help: Add Q_NAMESPACE_EXPORT to CMAKE_AUTOMOC_MACRO_NAMES default values

This was accidentally left out of commit 426941c4 (Autogen: Recognize
the new Q_NAMESPACE_EXPORT macro in AUTOMOC, 2020-02-26,
v3.17.0-rc2~3^2).

Help/variable/CMAKE_AUTOMOC_MACRO_NAMES.rst

@@ -11,7 +11,7 @@ This variable is used to initialize the :prop_tgt:`AUTOMOC_MACRO_NAMES`
 property on all the targets. See that target property for additional
 information.
 
-The default value is ``Q_OBJECT;Q_GADGET;Q_NAMESPACE``.
+The default value is ``Q_OBJECT;Q_GADGET;Q_NAMESPACE;Q_NAMESPACE_EXPORT``.
 
 Example
 ^^^^^^^