Install headers for the qt6 flavor in a unique location. It's silly that
they're still named qt5*.h, but we're trying not to deviate too much from
upstream and this interfaces's only consumer, kmymoney.

--- gui/qt5/Makefile.in.orig	2025-08-03 09:12:07 UTC
+++ gui/qt5/Makefile.in
@@ -556,7 +556,7 @@ nodist_noinst_HEADERS = 
 
 nodist_libgwengui_qt@QT_MAJOR_VERSION@_la_SOURCES = 
 nodist_noinst_HEADERS = 
-iheaderdir = ${gwenhywfar_headerdir_am}/gwen-gui-qt5
+iheaderdir = ${gwenhywfar_headerdir_am}/gwen-gui-qt@QT_MAJOR_VERSION@
 iheader_HEADERS = \
  qt5_gui.hpp \
  qt5_gui_dialog.hpp \
