Correct errno headers.

This should have no impact on tests, but addresses an incompatibility with
the standard library structure.

Author: lcartey

cpp/common/test/includes/standard-library/cerrno

@@ -1,2 +1 @@
-int __errno;
-#define errno __errno
+#include <errno.h>

cpp/common/test/includes/standard-library/errno.h

@@ -1,3 +1,5 @@
-namespace std {
-typedef int errno_t;
-} // namespace std
+#ifndef _GHLIBCPP_ERRNO
+#define _GHLIBCPP_ERRNO
+int __errno;
+#define errno __errno
+#endif // _GHLIBCPP_ERRNO