Remove unconditional include sys/poll.h

This fixes the build on windows (where other headers are included below)
and on fuchsia (where a warning tells us to use poll.h, included below,
instead of poll).

Change-Id: Ib3520d22353db245658828620eaffcb5f612b502

src/base/threading/channel_unittest.cc

diff --git a/src/base/threading/channel_unittest.cc b/src/base/threading/channel_unittest.cc
index 21c5ce7..0649678 100644
--- a/src/base/threading/channel_unittest.cc
+++ b/src/base/threading/channel_unittest.cc
@@ -15,7 +15,6 @@
  */
 
 #include "perfetto/ext/base/threading/channel.h"
-#include <sys/poll.h>
 #include <memory>
 
 #include "perfetto/base/platform_handle.h"