Remove tls.h inclusion from internal errno.h

The tls.h inclusion is not really required and limits possible
definition on more arch specific headers.

This is a cleanup to allow inline functions on sysdep.h, more
specifically on i386 and ia64 which requires to access some tls
definitions its own.

No semantic changes expected, checked with a build against all
affected ABIs.

sysdeps/unix/sysv/linux/generic/readlink.c

@@ -16,9 +16,9 @@
    License along with the GNU C Library.  If not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <errno.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <sysdep.h>
 
 /* Read the contents of the symbolic link PATH into no more than
    LEN bytes of BUF.  The contents are not null-terminated.