xfstests: Incorrect check for gdbm/ndbm.h

xfstests: Incorrect check for gdbm/ndbm.h

Current version of AC_PACKAGE_WANT_NDBM has following bugs:
* a typo (',') next to 'gdbm/ndbm.h', so C compiler fails
  with a syntax error when trying to compile
  "#include <gdbm/ndbm.h,>"

* autoconf never defines HAVE_GDBM_NDBM_H_ because it
  converts both header names (gdbm/ndbm.h, gdbm-ndbm.h)
  to GDBM_NDBM_H

Because of these bugs 'dbtest' can't be compiled on systems where
'gdbm-ndbm.h' header is absent but 'gdbm/ndbm.h' is present.

Fixed this.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>