BUILD: sock_inet: include errno.h

I was careful to have it for sock_unix.c but missed it for sock_inet
which broke with commit 36722d227 ("MINOR: sock_inet: report the errno
string in binding errors") depending on the build options. No backport
is needed.

src/sock_inet.c

@@ -10,6 +10,7 @@
  *
  */
 
+#include <errno.h>
 #include <fcntl.h>
 #include <string.h>
 #include <unistd.h>