sem_open.3: ATTRIBUTES: Note function that is thread-safe

The marking matches glibc marking.
The marking of functions in glibc is:
- sem_open: MT-Safe

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/sem_open.3

@@ -167,6 +167,19 @@ wasn't well formed.
 .TP
 .B ENOMEM
 Insufficient memory.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR sem_open ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 POSIX.1-2001.
 .SH SEE ALSO