Add missing include

Change-Id: I6d5a23852cf13fb84e072d34483d5e6e4d2e589d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/85807
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Lily Chen <chlily@google.com>
Commit-Queue: Lily Chen <chlily@google.com>

pki/merkle_tree.h

diff --git a/pki/merkle_tree.h b/pki/merkle_tree.h
index b764432..dae1726 100644
--- a/pki/merkle_tree.h
+++ b/pki/merkle_tree.h
@@ -19,6 +19,7 @@
 
 #include <array>
 #include <optional>
+#include <tuple>
 
 #include <openssl/sha2.h>
 #include <openssl/span.h>