Remove traditional `errno` usage

On modern POSIX environments, `errno` may not be simple `int` type,
and <errno.h> is sufficient for `errno` and its symbolic values.

Signed-off-by: OBATA Akio <obache@wizdas.com>
(cherry picked from commit 35e5ff35cf)

mpg123_artsplugin/mpg123/httpget.c

@@ -18,11 +18,9 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
-#include <sys/errno.h>
+#include <errno.h>
 #include <ctype.h>
 
-extern int errno;
-
 #include "mpg123.h"
 
 #ifndef INADDR_NONE