[fuchsia] Add missing #include.

Change-Id: Ie11606d71cd35269cce836407d9a302f45602f2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2479368
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817981}

fuchsia/base/context_provider_test_connector.cc

@@ -8,6 +8,7 @@
 
 #include <fuchsia/sys/cpp/fidl.h>
 #include <lib/fdio/directory.h>
+#include <lib/fdio/fd.h>
 #include <lib/sys/cpp/component_context.h>
 #include <zircon/processargs.h>
 #include <utility>