Add missing header for using with std::move

Compilation with gcc is outputing this error:

no member named 'move' in namespace 'std'

Change-Id: Idde1562a9ea9581816b35323a182ddda5c0aea12
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/7720
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

src/gn/hash_table_base.h

diff --git a/src/gn/hash_table_base.h b/src/gn/hash_table_base.h
index 9e43733..22fcaea 100644
--- a/src/gn/hash_table_base.h
+++ b/src/gn/hash_table_base.h
@@ -9,6 +9,7 @@
 
 #include <stdlib.h>
 #include <type_traits>
+#include <utility>
 
 // IMPORTANT DISCLAIMER:
 //