cxx_callback_datatype test case: fix build error on windows
diff --git a/tests/cxx_callback_datatype/cxx_callback_datatype.cpp b/tests/cxx_callback_datatype/cxx_callback_datatype.cpp
index 52f2742..f95b4f6 100644
--- a/tests/cxx_callback_datatype/cxx_callback_datatype.cpp
+++ b/tests/cxx_callback_datatype/cxx_callback_datatype.cpp
@@ -3,6 +3,7 @@
 #include <pb_encode.h>
 #include <pb_decode.h>
 
+#include <algorithm>
 #include <cstdio>
 
 // See tests/alltypes_callback, tests/oneoff_callback and examples/network_server for more...