Fix build failures with --disable-shadowgrp. Thanks to Jürgen

Daubert for the patch.
* libmisc/salt.c: Include <stdio.h>, needed for stderr and printf
  functions.
* lib/encrypt.c: Include <stdio.h>, needed for perror, stderr and
  printf functions
* src/usermod.c: sgr_locked exists only if SHADOWGRP is defined.
* src/chgpasswd.c: Only check is the gshadow file exists if
  SHADOWGRP is defined.

libmisc/salt.c

@@ -13,6 +13,7 @@
 
 #include <sys/time.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <assert.h>
 #include "prototypes.h"
 #include "defines.h"