RenderWidget: Add include to Windows.h

[RA-Kooi]

Fixes errors when building with CMake (Ninja).
Currently it depends on PCH support, but it's broken currently.
Not depending on PCH is good practice anyway.

Not sure if Windows.h is the most lightweight include though, but it at least fixes the error of there not being a target defined.

[Pokechu22]

The specific error:

9>RenderWidget.cpp
9>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22000.0\um\winnt.h(169,1): fatal error C1189: #error:  "No Target Architecture"
9>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22000.0\um\winnt.h(169,1): fatal error C1189: #error "No Target Architecture"
9>C:\Program Files (x86)\Windows Kits\10\Include\10.0.22000.0\um\winnt.h(169,1): fatal error C1189: ^

According to this SO post you can just include windows.h on its own, and that includes windef.h. Note that Dolphin defines WIN32_LEAN_AND_MEAN so windows.h is much more lightweight.

As a related note, Dolphin seems to be inconsistent about specifying windows.h vs Windows.h (but that doesn't matter since Windows uses a case-insensitive filesystem; you can even use wInDoWs.H). The current inconsistency isn't something you need to deal with, and it's probably best to stick with Windows.h since that file also uses WinUser.h instead of winuser.h.