getifaddrs.3: ATTRIBUTES: Note functions that are thread-safe

After research, We think getifaddrs() and freeifaddrs() are not
thread-safe. But, there is not markings of getifaddrs() and
freeifaddrs() in glibc document.
- getifaddrs:  MT-Safe
- freeifaddrs: MT-Safe

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

man3/getifaddrs.3

@@ -167,6 +167,20 @@ the implementation supported only IPv4 addresses;
 IPv6 support was added in glibc 2.3.3.
 Support of address families other than IPv4 is available only
 on kernels that support netlink.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw27 lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR getifaddrs (),
+.BR freeifaddrs ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 Not in POSIX.1-2001.
 This function first appeared in BSDi and is