Fix gcc compilation by including required header

Compilation with gcc is currently failing with the
following error:
```
error: no template named 'equal_to' in namespace 'std'
```

Change-Id: Ie1a705b97bbbd6bf0615e176af29f9d9a0dc4467
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/12741
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

AUTHORS

diff --git a/AUTHORS b/AUTHORS
index 0c64ad6..45cedd7 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -15,6 +15,7 @@
 MIPS Technologies, Inc. <*@mips.com>
 NVIDIA Corporation <*@nvidia.com>
 Opera Software ASA <*@opera.com>
+Red Hat Inc. <*@redhat.com>
 The Chromium Authors <*@chromium.org>
 Vewd Software AS <*@vewd.com>
 Vivaldi Technologies AS <*@vivaldi.com>

src/gn/unique_vector.h

diff --git a/src/gn/unique_vector.h b/src/gn/unique_vector.h
index da4edc2..0f6994f 100644
--- a/src/gn/unique_vector.h
+++ b/src/gn/unique_vector.h
@@ -9,6 +9,7 @@
 #include <stdint.h>
 
 #include <algorithm>
+#include <functional>
 #include <vector>
 
 #include "hash_table_base.h"