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.

src/chgpasswd.c

@@ -342,7 +342,9 @@ int main (int argc, char **argv)
 
 	check_perms ();
 
+#ifdef SHADOWGRP
 	is_shadow_grp = sgr_file_present ();
+#endif
 
 	open_files ();