system_data_types.7: Add 'clock_t'

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/system_data_types.7

@@ -85,6 +85,31 @@ See also:
 .BR aio_write (3),
 .BR lio_listio (3)
 .RE
+.\"------------------------------------- clock_t ----------------------/
+.TP
+.I clock_t
+.RS
+Include:
+.I <time.h>
+or
+.IR <sys/types.h> .
+Alternatively,
+.IR <sys/time.h> .
+.PP
+Used for system time in clock ticks or
+.B CLOCKS_PER_SEC
+(defined in
+.IR <time.h> ).
+According to POSIX,
+it shall be an integer type or a real-floating type.
+.PP
+Conforming to:
+C99 and later; POSIX.1-2001 and later.
+.PP
+See also:
+.BR times (2),
+.BR clock (3)
+.RE
 .\"------------------------------------- div_t ------------------------/
 .TP
 .I div_t