cpp-common/bt2c: use `ComparableWithCStringView` on `operator!=` meant for `CStringView`

cpp-common/bt2c: use `ComparableWithCStringView` on `operator!=` meant for `CStringView`

I don't recall exactly when I needed this, but it occured to me once
that the `operator!=` we provide for `CStringView` was getting picked up
for some unrelated type.  It just seems dangerous to provide a generic
templated operator like that that can match literally anything.

Use the same `ComparableWithCStringView` conditions here that we use for
`operator==` just above, so that it only gets used for what it was
intended for.

Change-Id: I798f1c8195c541f6188499dcfabfe1233a95a492
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11962
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>