binary_messenger missing cstdint (#48481)

This PR adds cstdint to binary_messenger.h.  Without this building plugin support for generic Linux fails.  Not related to the GTK source tree.

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/common/client_wrapper/include/flutter/binary_messenger.h

diff --git a/shell/platform/common/client_wrapper/include/flutter/binary_messenger.h b/shell/platform/common/client_wrapper/include/flutter/binary_messenger.h
index d552b49..553ba0b 100644
--- a/shell/platform/common/client_wrapper/include/flutter/binary_messenger.h
+++ b/shell/platform/common/client_wrapper/include/flutter/binary_messenger.h
@@ -5,6 +5,7 @@
 #ifndef FLUTTER_SHELL_PLATFORM_COMMON_CLIENT_WRAPPER_INCLUDE_FLUTTER_BINARY_MESSENGER_H_
 #define FLUTTER_SHELL_PLATFORM_COMMON_CLIENT_WRAPPER_INCLUDE_FLUTTER_BINARY_MESSENGER_H_
 
+#include <cstdint>
 #include <functional>
 #include <string>