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/sysctl.c

@@ -17,6 +17,7 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
+#include <stddef.h>
 #include <shlib-compat.h>
 
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_32)