Guard pipewire configs with rtc_use_pipewire.
Guard pipewire configs with rtc_use_pipewire.

[1] has introduced two dependencies on
//modules/desktop_capture:pipewire_config
even when rtc_use_pipewire=false, this CL changes the guard in order to
make sure GN doesn't raise errors when is_linux=true and
rtc_use_pipewire=false.

[1] - https://webrtc-review.googlesource.com/c/src/+/160649

No-Try: True
Bug: chromium:682122
Change-Id: I28d2f10936dd75199a2a98862751708eb1e5615a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187122
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Reviewed-by: Tommi <74a97831ec830b6e3846c6faa50560947ae01c68@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#32345}

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