lttng-libust: bump to version 2.8.1

The two current patches are removed because they were merged upstream.

The new patch is needed to build with uClibc. It was also submitted
upstream, but not merged as is yet. Defining NT_GNU_BUILD_ID to 3 when
it's not defined seems like it's something done in other projects. This
definition's value should never change (number of the build ID note
section in ELF objects).

LTTng-UST now has its man pages written in AsciiDoc and they get
converted to troff at build time. This is not needed in a Buildroot
image anyway.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>