generate_ech.cc: include needed headers

Change-Id: I04c8bb68801aeb0938e5b038b98811ca4ffe50f0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48685
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

tool/generate_ech.cc

diff --git a/tool/generate_ech.cc b/tool/generate_ech.cc
index 1fe501d..f1e8cbe 100644
--- a/tool/generate_ech.cc
+++ b/tool/generate_ech.cc
@@ -14,6 +14,7 @@
 
 #include <stdio.h>
 
+#include <limits>
 #include <vector>
 
 #include <openssl/bytestring.h>