libc/passwd: add pw_gecos field(userinfo)

The pw_gecos field is not specified in POSIX, but is
present on most implementations.
passwd file format:
name:uid:gid:gecos:dir:shell
ex: gdm:x:127:133:Gnome Display Manager:/var/lib/gdm3:/bin/false

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

include/pwd.h

@@ -49,6 +49,7 @@ struct passwd
   FAR char *pw_name;
   uid_t     pw_uid;
   gid_t     pw_gid;
+  FAR char *pw_gecos;
   FAR char *pw_dir;
   FAR char *pw_shell;
 };