import-checker: force 'ctype.util' to stdlib module

Not having this caused warnings on Windows:

  mercurial/pure/osutil.py:12: stdlib import follows local import: os
  mercurial/pure/osutil.py:13: stdlib import follows local import: socket
  mercurial/pure/osutil.py:14: stdlib import follows local import: stat
  mercurial/pure/osutil.py:15: stdlib import follows local import: sys

contrib/import-checker.py

@@ -180,7 +180,7 b' def list_stdlib_modules():'
     for m in ['msvcrt', '_winreg']:
         yield m
     # These get missed too
-    for m in 'ctypes', 'email', 'multiprocessing':
+    for m in 'ctypes', 'ctypes.util', 'email', 'multiprocessing':
         yield m
     yield 'builtins' # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only