rational.c: conditionally defined variable

* rational.c (numeric_quo): variable 'canonicalization' is defined
  conditionally by CANON (and CANONICALIZATION_FOR_MATHN).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e