fix IWYU

src/replica/split/test/replica_split_test.cpp

@@ -15,8 +15,9 @@
 // specific language governing permissions and limitations
 // under the License.
 
-#include <stdint.h>
 #include <atomic>
+#include <cstddef>
+#include <cstdint>
 #include <memory>
 #include <string>
 #include <unordered_set>