Fix: lttng-ust-elf.c: define NT_GNU_BUILD_ID if not defined

Fix: lttng-ust-elf.c: define NT_GNU_BUILD_ID if not defined

On uClibc, NT_GNU_BUILD_ID is not defined, so we define it
manually in this case.

This definition is the number given to the .note.gnu.build-id section
(of type SHT_NOTE) of a linked ELF object.

    The ELF note headers give name "GNU" and type 3 (NT_GNU_BUILD_ID)
for a build ID note, of which there can be only one in a linked object
[...]

See http://fedoraproject.org/wiki/RolandMcGrath/BuildID

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>