sha2.c: suppress warnings

* ext/digest/sha2/sha2.c (SHA256_Final, SHA512_Last): suppress
  strict-aliasing warnings on gcc 4.8.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40033 b2dd03c8-39d4-4d8f-98ff-823fe69b080e