[C++][FS] Actually enable GCS support in FileSystemFromUri()

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

Related issues:

• [Python] Add bindings for GCS filesystem (is duplicated by)

PRs and other links:

• GitHub Pull Request #12932

_{Note: This issue was originally created as ARROW-16248. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
ARROW-14892 will also fix this.