Add GNU/Hurd hints for NDBM_File

currently, there are few tests failing because of a broken NDBM module;
using the same hints as used on Linux (given that apparently the cause
is the same) makes the following tests pass:
  t/op/dbm.t
  cpan/autodie/t/dbmopen.t
  cpan/Memoize/t/errors.t
  cpan/Memoize/t/tie_ndbm.t
  ext/NDBM_File/t/ndbm.t
  lib/AnyDBM_File.t

Dominic Hargreaves adds: Note that the original git commit log [what
you see above] lists some tests which aren't affected in blead.

Author: Pino Toscano

MANIFEST

@@ -3679,6 +3679,7 @@ ext/mro/mro.xs			mro extension
 ext/NDBM_File/hints/cygwin.pl	Hint for NDBM_File for named architecture
 ext/NDBM_File/hints/dec_osf.pl	Hint for NDBM_File for named architecture
 ext/NDBM_File/hints/dynixptx.pl	Hint for NDBM_File for named architecture
+ext/NDBM_File/hints/gnu.pl	Hint for NDBM_File for named architecture
 ext/NDBM_File/hints/gnukfreebsd.pl	Hint for NDBM_File for named architecture
 ext/NDBM_File/hints/gnuknetbsd.pl	Hint for NDBM_File for named architecture
 ext/NDBM_File/hints/linux.pl	Hint for NDBM_File for named architecture

ext/NDBM_File/hints/gnu.pl

@@ -0,0 +1 @@
+do './hints/linux.pl' or die $@;