Add change log about source incompatibility in moc output

"Use std::addressof for taking an address instead of operator& in moc"
(5b99f3a) introduced a dependency to
std memory header. That change exposed code that was including
the moc output from within a namespace. That was never supported nor
valid, but it used to compile. The problem was not detected earlier
because both other includes where already included to support QObject
itself and therefore inside the moc output their header guards were
disabling their content.

Task-number: QTBUG-68191
Task-number: QTBUG-70907
Change-Id: I18462ed17d748a114fe2e5b6bf9aeb56ee26e2db
Reviewed-by: Frederik Gladhorn <[email protected]>
Reviewed-by: Timur Pocheptsov <[email protected]>
Reviewed-by: Edward Welbourne <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>

dist/changes-5.13.0

@@ -0,0 +1,7 @@
+****************************************************************************
+*                 Important Source Incompatible Changes                    *
+****************************************************************************
+
+ - Moc generated files include the standard <memory> header file now.
+   A side effect of this is that code including the Moc output from within
+   a namespace will fail to compile.