Add build_config.h to unittest file (fix presubmit warning)

Bug: 946001
Change-Id: I9131bb30d774cd38d169cb92d41fd357cdf0d71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040000
Auto-Submit: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738705}

remoting/host/desktop_display_info_unittest.cc

@@ -5,6 +5,7 @@
 #include "remoting/host/desktop_display_info.h"
 
 #include "base/location.h"
+#include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 // TODO(crbug.com/961064): Fix memory leaks in tests and re-enable on LSAN.