Include C++ <cerrno> instead of C <errno.h>

c++/src/capnp/compat/json.c++

@@ -22,7 +22,7 @@
 #include "json.h"
 #include <cmath>    // for HUGEVAL to check for overflow in std::strtod
 #include <cstdlib>  // std::strtod
-#include <errno.h>  // for std::strtod errors
+#include <cerrno>   // for std::strtod errors
 #include <unordered_map>
 #include <capnp/orphan.h>
 #include <kj/debug.h>