Replace RUBY_EXPORT with RUBY_FUNC_EXPORTED since it's the recommended way

[maxprokopiev]

RUBY_EXPORT used by ruby internally to figure out if library is static or dynamic. The change in https://github.com/ruby/ruby/pull/9828 broke rinku because it couldn't find ruby_abi_version() anymore.

RUBY_FUNC_EXPORTED is recommended in that case (see https://github.com/ruby/ruby/blob/b2392c6be418703e8941226ac80b359188bf3c5d/lib/bundler/templates/newgem/ext/newgem/extconf-c.rb.tt#L6)