Fix -Wjump-misses-init warnings

Fix -Wjump-misses-init warnings

Fix gcc warnings of this kind:

      CC       stream-class.lo
    /home/simark/src/babeltrace/src/ctf-writer/stream-class.c: In function ‘bt_ctf_stream_class_common_add_event_class’:
    /home/simark/src/babeltrace/src/ctf-writer/stream-class.c:177:3: error: jump skips variable initialization [-Werror=jump-misses-init]
      177 |   goto end;
          |   ^~~~
    /home/simark/src/babeltrace/src/ctf-writer/stream-class.c:389:1: note: label ‘end’ defined here
      389 | end:
          | ^~~
    /home/simark/src/babeltrace/src/ctf-writer/stream-class.c:241:29: note: ‘query’ declared here
      241 |  struct bt_ctf_search_query query = { .value = event_class, .found = 0 };
          |                             ^~~~~

Fix it by moving the declarations near the top, as our coding style
prescribes.

Change-Id: I49209e40894a362f84c413e50640ea62ff040de4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2267
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>