Back port [1] to fix build with Qt 6.10.

[1] https://github.com/PCSX2/pcsx2/commit/8dffc857079e942ca77b091486c20c3c6530e4ed

--- pcsx2-qt/CMakeLists.txt.orig	2025-06-29 19:47:12 UTC
+++ pcsx2-qt/CMakeLists.txt
@@ -265,6 +265,7 @@ target_link_libraries(pcsx2-qt PRIVATE
 	PCSX2
 	Qt6::Core
 	Qt6::Gui
+	Qt6::GuiPrivate
 	Qt6::Widgets
 	KDAB::kddockwidgets
 )