Add missing include of unistd.h

Fixup after the change in cl
https://webrtc-review.googlesource.com/c/src/+/109003

Bug: webrtc:6424
Change-Id: I4aff8557a895804147d5646ce916818cca90d3b5
Reviewed-on: https://webrtc-review.googlesource.com/c/110723
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Max Morin <maxmorin@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25616}

rtc_base/filerotatingstream.cc

@@ -21,6 +21,7 @@
 #else
 #include <dirent.h>
 #include <sys/stat.h>
+#include <unistd.h>
 #endif  // WEBRTC_WIN
 
 #include "absl/strings/match.h"