Add dxva.h include to get GUIDs

In order to build supported_profile_helpers.cc with the Windows 10 20H1
(2004, aka 19041) SDK it is necessary to include dxva.h in order to get
the GUID definitions.

This was found while working on crrev.com/c/2225224. I'm splitting it
out into a separate CL because this change is clean and obvious,
whereas other changes are more like hacks and workarounds.

Bug: 1089996
Change-Id: I4f594b918551f10f90676481bf5336731bd61df6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227946
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774808}

media/gpu/windows/av1_guids.h

@@ -5,6 +5,7 @@
 #ifndef MEDIA_GPU_WINDOWS_AV1_GUIDS_H_
 #define MEDIA_GPU_WINDOWS_AV1_GUIDS_H_
 
+#include <dxva.h>
 #include <initguid.h>
 
 #if WDK_NTDDI_VERSION <= NTDDI_WIN10_19H1