lib: internal: define `BT_ASSERT_{PRE,POST}_SUPPORTED` in headers

lib: internal: define `BT_ASSERT_{PRE,POST}_SUPPORTED` in headers

This is to make some headers require that precondition and postcondition
assertion macros exist when including them, just like
`src/lib/logging.h` defines `BT_LIB_LOG_SUPPORTED` for the same reason.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ia2db609ae540b29129b9260c237f8b8d9657d977
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1576
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>