Make libqt6gui6 recommend qt6-qpa-plugins.

That package contains libqxcb.so, without which Qt will not work on
X11 platform.

debian/changelog

@@ -1,5 +1,8 @@
 qt6-base (6.3.1+dfsg-6) UNRELEASED; urgency=medium
 
+  [ Dmitry Shachnev ]
+  * Make libqt6gui6 recommend qt6-qpa-plugins. That package contains
+    libqxcb.so, without which Qt will not work on X11 platform.
 
  -- Debian Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>  Sun, 24 Jul 2022 19:36:07 +0300
 

debian/control

@@ -128,7 +128,7 @@ Pre-Depends: ${misc:Pre-Depends}
 Depends: fontconfig, ${misc:Depends}, ${shlibs:Depends}
 Breaks: qt6-qpa-plugins (<< 6.2.4+dfsg-5~)
 Replaces: qt6-qpa-plugins (<< 6.2.4+dfsg-5~)
-Recommends: qt6-gtk-platformtheme
+Recommends: qt6-gtk-platformtheme, qt6-qpa-plugins
 Description: Qt 6 GUI module
  Qt is a cross-platform C++ application framework. Qt's primary feature
  is its rich set of widgets that provide standard GUI functionality.