Fix: Move generated headers to the BUILT_SOURCES target

This fixes the out of tree build when enabling Java support. The
problem in using an LTTngUst.c target is that automake will then
assume the file is generated and thus be found in the builddir.

Using the BUILT_SOURCES target will ensure the
org_lttng_ust_LTTngUst.h and org_lttng_ust_jul_LTTngUst.h headers
are generated before LTTngUst.c is compiled.

Tested-by: David Goulet <dgoulet@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>