Fix: build against Ubuntu kernels

Fix: build against Ubuntu kernels

Recent Ubuntu kernels appear to define DEBIAN_API_VERSION, but they are
not really a Debian kernel per-se: the abi-debian-version.sh cannot find
the expected Debian kernel version string layout.

Therefore, change the conditional check to use directly the
LTTNG_DEBIAN_VERSION_CODE define, which is defined only when the
abi-debian-version.sh script figures out it builds against a Debian
kernel.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>