Fix: Add --no-as-needed to the demo example's Makefile

Some distributions now ship with the --as-needed linker flag
set by default (Ubuntu 13.04). This will cause the linker to
remove the references to lttng-ust from the provider objects
thus causing the application to fail when preloading them.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>