[libc++] Add missing <errno.h> include in threading support headers (#80311)

This was incorrectly removed when I split up the header.

Author: ldionne

libcxx/include/__thread/support/c11.h

@@ -14,6 +14,7 @@
 #include <__chrono/duration.h>
 #include <__config>
 #include <ctime>
+#include <errno.h>
 #include <threads.h>
 
 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER