bignum.c: suppress warnings

* bignum.c (integer_unpack_num_bdigits_generic): suppress "implicit
  conversion shortens 64-bit value into a 32-bit value" warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41274 b2dd03c8-39d4-4d8f-98ff-823fe69b080e