change errno.h to cerrno

The former is deprecated by C++14. It's also functionally the same.

From the standard:

19.4
The header<cerrno>is described in Table 43. Its contents are the same as
the POSIX header<errno.h>,except that errno shall be defined as a macro.
[Note: The intent is to remain in close alignment with the POSIX
standard.] A separate errno value shall be provided for each thread.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

test/WriteFile.cxx

@@ -20,8 +20,9 @@
 #include "fs/io/FileOutputStream.hxx"
 #include "util/PrintException.hxx"
 
+#include <cerrno>
+
 #include <unistd.h>
-#include <errno.h>
 
 #include <string.h>
 #include <stdlib.h>