Include <functional> in third_party/base/optional.h.

optional.h defines struct hash<base::Optional<T>>, and struct hash comes
from <functional>. This is the PDFium equivalent to
https://crrev.com/c/1536740

Change-Id: Ice770a13e29d132adacf21d64ea048750c2b7b9c
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/52291
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/base/optional.h b/third_party/base/optional.h
index f98ee7e..5998105 100644
--- a/third_party/base/optional.h
+++ b/third_party/base/optional.h
@@ -5,6 +5,7 @@
 #ifndef THIRD_PARTY_BASE_OPTIONAL_H_
 #define THIRD_PARTY_BASE_OPTIONAL_H_
 
+#include <functional>
 #include <type_traits>
 #include <utility>