use do/while(0) around GetDBM macros

* README.EXT: wrap GetDBM with do/while(0)
* README.EXT.ja: ditto
* ext/dbm/dbm.c: ditto, likewise for GetDBM2
* ext/gdbm/gdbm.c: ditto
* ext/sdbm/init.c: ditto
  [ruby-core:61217]

ref: http://c-faq.com/cpp/multistmt.html

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e