DolphinQt: Fix generation of /external:I flags

Mainly concerns to building with Ninja, as that's what I tested it with.
Originally it would only prepend the first path with `/external:I`,
however all paths in the list have to be prepended with `/external:I`.
The MS documentation seems to support this, as it makes no mention of it
accepting a list.

This is probably the worst way to implement this, I am unfamiliar with
CMake.

Source/Core/DolphinQt/CMakeLists.txt

@@ -369,8 +369,15 @@ if (MSVC)
   # Don't propogate warnings in qt headers to Dolphin
   target_compile_options(dolphin-emu PRIVATE /experimental:external)
   target_compile_options(dolphin-emu PRIVATE /external:W0)
-  target_compile_options(dolphin-emu PRIVATE "/external:I${Qt5Gui_PRIVATE_INCLUDE_DIRS}")
-  target_compile_options(dolphin-emu PRIVATE "/external:I${Qt5Widgets_PRIVATE_INCLUDE_DIRS}")
+  set(qtGui "")
+  set(qtGuiPriv "")
+  set(qtWidgetsPriv "")
+  list(TRANSFORM Qt5Gui_INCLUDE_DIRS PREPEND "/external:I" OUTPUT_VARIABLE qtGui)
+  list(TRANSFORM Qt5Gui_PRIVATE_INCLUDE_DIRS PREPEND "/external:I" OUTPUT_VARIABLE qtGuiPriv)
+  list(TRANSFORM Qt5Widgets_PRIVATE_INCLUDE_DIRS PREPEND "/external:I" OUTPUT_VARIABLE qtWidgetsPriv)
+  target_compile_options(dolphin-emu PRIVATE "${qtGui}")
+  target_compile_options(dolphin-emu PRIVATE "${qtGuiPriv}")
+  target_compile_options(dolphin-emu PRIVATE "${qtWidgets}")
 endif()
 
 if(WIN32)