arrow

GH-44601: [GLib] Fix the wrong GARROW_AVAILABLE_IN declaration (#44602)



### Rationale for this change

GArrowDecimal64 will be introduced in the 19.0.0 release.
This part should be `GARROW_AVAILABLE_IN_19_0`
instead of `GARROW_AVAILABLE_IN_ALL`

### What changes are included in this PR?

Change the `GARROW_AVAILABLE_IN` declaration.

### Are these changes tested?

YES

### Are there any user-facing changes?

NO

* GitHub Issue: #44601

Authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>