Fix compilation with gcc-15.
Bug: https://bugs.gentoo.org/946146
Upstream PR: https://github.com/open-telemetry/opentelemetry-cpp/pull/3240
--- a/api/include/opentelemetry/logs/severity.h
+++ b/api/include/opentelemetry/logs/severity.h
@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <cstdint>
+
 #include "opentelemetry/nostd/string_view.h"
 #include "opentelemetry/version.h"