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

The marking matches glibc marking.
The marking of functions in glibc is:
- __fpurge: MT-Safe race:stream

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

man3/fpurge.3

@@ -67,6 +67,19 @@ appropriately.
 .B EBADF
 .I stream
 is not an open stream.
+.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 __fpurge ()
+T}	Thread safety	MT-Safe race:stream
+.TE
+
 .SH CONFORMING TO
 These functions are nonstandard and not portable.
 The function