clock.3: ATTRIBUTES: Note functions that is thread-safe

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

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/clock.3

@@ -47,6 +47,19 @@ to get the number of seconds used, divide by
 If the processor time used is not available or its value cannot
 be represented, the function returns the value
 .IR (clock_t)\ \-1 .
+.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 clock ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 C89, C99, POSIX.1-2001.
 XSI requires that