Fix declaration of snprintf for older MSVC

_snprintf_s takes an additional (size) argument, so is no direct replacement.
(Note that this code is currently unused - the two instances of snprintf here are within ifdef blocks that are not compiled for MSVC)

utest/ctest.h

@@ -84,7 +84,7 @@ struct ctest {
 #endif
 
 #if _MSC_VER < 1900
-#define snprintf _snprintf_s
+#define snprintf _snprintf
 #endif
 
 #ifndef __cplusplus