Fix <utmp.h> declarations.

If we're going to keep this useless header -- and I think it's too late to
remove it -- it may as well be correct.

Bug: http://b/17700469
Change-Id: Ifec4f8f1a984483d7fa7d81d47786f75b70ff4ba

libc/include/utmp.h

@@ -97,8 +97,9 @@ struct utmp
 __BEGIN_DECLS
 
 int utmpname(const char*);
-void setutent();
-struct utmp* getutent();
+void setutent(void);
+struct utmp* getutent(void);
+void endutent(void);
 
 int login_tty(int);