Define enable_extension method to prevent undefined method error

When mixing postgresql and another adapter like sqlite3 (for dev and test
respectively), the task `db:test:prepare` will fail due to the `enable_extension`
method not being defined in the abstract adapter. This patch simply adds an
empty definition to prevent it.
Merge pull request #11455 from antonio/fix_enable_extension_not_being_defined

Define enable_extension method to prevent undefined method error