Don't build windows core audio if using dummy file devices.
Don't build windows core audio if using dummy file devices.

If WEBRTC_DUMMY_FILE_DEVICES is set, WEBRTC_CORE_AUDIO_BUILD should not.
Otherwise audio_device_core_win.h will be included [1] when it shouldn't
(according to [2]).

[1] https://webrtc.googlesource.com/src/+/master/modules/audio_device/audio_device_impl.cc#22
[2] https://webrtc.googlesource.com/src/+/master/modules/audio_device/BUILD.gn#177

Bug: webrtc:6265
Change-Id: Ia6ccb9dda39f411c0d8a548a0501408e87d11a40
Reviewed-on: https://webrtc-review.googlesource.com/16430
Reviewed-by: Oleh Prypin <5baa3e04f9bd6ba7a2a4b56625a3c84f19237cdb@webrtc.org>
Reviewed-by: Henrik Andreassson <e468f06af92d21183a2a635f545ff5cefc8c8b65@webrtc.org>
Commit-Queue: Edward Lemur <d05609f027dc667cc37492b1dc1bdea5d60fa82a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20464}
ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc