fgets.3: document gets_s() __STDC_WANT_LIB_EXT1__

This matches the man page for qsort_s().

PR:		281828
Signed-off-by:	Graham Percival <gperciva@tarsnap.com>
Sponsored by:	Tarsnap Backup Inc.

lib/libc/stdio/fgets.3

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 2, 2022
+.Dd December 6, 2024
 .Dt FGETS 3
 .Os
 .Sh NAME
@@ -42,6 +42,7 @@
 .In stdio.h
 .Ft char *
 .Fn fgets "char * restrict str" "int size" "FILE * restrict stream"
+.Fd #define __STDC_WANT_LIB_EXT1__ 1
 .Ft char *
 .Fn gets_s "char *str" "rsize_t size"
 .Sh DESCRIPTION