Fix macro missing from C++.

tests/cksum_read.c

@@ -11,6 +11,10 @@
 #include "var_cksum.h"
 #include "stream_cksum.h"
 
+#ifdef __cplusplus
+#define PRIx32 ""
+#endif
+
 static struct cksum_table *
 read_table(const char *table_fname, size_t *table_len)
 {