BUILD: sock_unix: add missing errno.h

It builds fine when openssl is enabled, but fails otherwise. No backport
is needed.
diff --git a/src/sock_unix.c b/src/sock_unix.c
index 8bdfed9..cf3e49c 100644
--- a/src/sock_unix.c
+++ b/src/sock_unix.c
@@ -10,8 +10,9 @@
  *
  */
 
-#include <fcntl.h>
 #include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <string.h>
 #include <unistd.h>