gtkprintoperation-win32: fix the build on Windows with GCC 14.1.0

Both cases expect a DLGTEMPLATE but the docs say that DLGTEMPLATEEX works too, despite it being a different struct, so just cast it. See https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-createdialogindirectw

This avoids two "-Wincompatible-pointer-types" which GCC 14 switched to being an error by default.