Cleanup: debug-info: move all `_FIELD_NAME` defines to the same file

Cleanup: debug-info: move all `_FIELD_NAME` defines to the same file

The `VPID_FIELD_NAME` and `IP_FIELD_NAME` are currently defined in both
`debug_info.c` and `debug-info.h`.

This commit regroups all `_FIELD_NAME` defines to the same file for
consistency.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I5afd805506beb91512cb855566c475d03ed333f6
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3615
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>