Silence unused variable warnings caused by BT_ASSERT() in non-debug mode

Silence unused variable warnings caused by BT_ASSERT() in non-debug mode

When building with the BABELTRACE_DEBUG_MODE configure option _not_ set,
multiple set-but-not-used warnings are emitted by the compiler. Most of
the time this happens because BT_ASSERT() is used to verify the return
value of functions and when not in DEBUG_MODE those return value are not
used.

The solution is to explicitly tell the compiler that we don't care about
this value so that it does not emit warning when building in non-debug
mode. This approach also makes sure not to evaluate the expression using
the `sizeof` operator, thus preventing any side effects.

See this post for further details [1].

[1]: https://stackoverflow.com/questions/37411809/how-to-elegantly-fix-this-unused-variable-warning/37412551#37412551

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>