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.

posix/execl.c

@@ -19,6 +19,7 @@
 #include <errno.h>
 #include <stdarg.h>
 #include <sys/param.h>
+#include <stddef.h>
 
 /* Execute PATH with all arguments after PATH until
    a NULL pointer and environment from `environ'.  */