Ensure `DDD_GID` is at least 64 bits wide

a0cf9c01 increased `MAX_PROCBITS_IN_GID`
from 9 to 24.  As an `unsigned long` is only 32 bits wide on common
32 bit architectures, this left only 8 bits instead of the intended
40 bits (on 64 bit architectures) for objects.

This patch makes sure the `DDD_GID` type is at least 64 bits by using
the `std::uint_least64_t` type.  It should fix the build failure on, for
example, [i386] and [PowerPC].

  [i386]: https://buildd.debian.org/status/fetch.php?pkg=dune-grid&arch=i386&ver=2.5.0%7Erc1-1&stamp=1480684720
  [PowerPC]: https://buildd.debian.org/status/fetch.php?pkg=dune-grid&arch=s390x&ver=2.5.0%7Erc1-1&stamp=1480658113

See merge request !38