Fix missing "#include" causing GitHub Actions failure (#92)

Summary:
Pull Request resolved: #92

fbjni OSS tests will build a binary using `ubuntu-latest` Clang + stdlibc++ toolchain. https://github.com/facebookincubator/fbjni/blob/main/scripts/run-host-tests.sh

The stdlib version changed, and now we fail to build, because `#include <string>` or another header is no longer pulling in `<cstdint>` for us.

Reviewed By: cipolleschi

Differential Revision: D51146182

fbshipit-source-id: 1bc532b36b8acd0f54b454cc9fcc90a9bd4cf245

cxx/fbjni/detail/utf8.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <cstdint>
 #include <string>
 
 #include <jni.h>