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/generic/internal-signals.h

@@ -22,6 +22,7 @@
 #include <signal.h>
 #include <sigsetops.h>
 #include <stdbool.h>
+#include <stddef.h>
 
 static inline bool
 __is_internal_signal (int sig)