(main): Declare to return int, not void.

src/who-users.c

@@ -644,7 +644,7 @@ If FILE not given, uses /etc/utmp.  /etc/wtmp as FILE is common.\n\
 }
 #endif /* USERS || UPTIME */
 
-void
+int
 main (int argc, char **argv)
 {
   int optc, longind;

src/whoami.c

@@ -61,7 +61,7 @@ Same as id -un.\n\
   exit (status);
 }
 
-void
+int
 main (int argc, char **argv)
 {
   register struct passwd *pw;