strfmon.3: Cleanup example code

- xlocale.h would have been required if using strfmon_l().  Here,
  setlocale() just requires locale.h.
- ANSIfy function declaration.
- Add a final return().

MFC after:	1 week

lib/libc/stdlib/strfmon.3

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd January 25, 2023
+.Dd December 6, 2023
 .Dt STRFMON 3
 .Os
 .Sh NAME
@@ -154,10 +154,10 @@ to the string
 .Bd -literal -offset indent
 #include <stdio.h>
 #include <monetary.h>
-#include <xlocale.h>
+#include <locale.h>
 
 int
-main()
+main(void)
 {
 	char string[100];
 	double money = 1234567.89;
@@ -169,6 +169,8 @@ main()
 
 	strfmon(string, sizeof(string) - 1, "%n", money);
 	printf("%s\\n", string);
+
+	return (0);
 }
 .Ed
 .Sh ERRORS