lib: internal: graph.h: require logging and BT_ASSERT_{PRE,POST}()

lib: internal: graph.h: require logging and BT_ASSERT_{PRE,POST}()

This patch makes the internal `graph.h` require what it needs instead of
using the macros only if they exist. This makes it impossible to skip
logging or precondition/postcondition validation in this header.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I157809be7b56720b2f4a241c45491413f5b8366d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1577
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>