lttng-libust: Disable liblttng-ust-dl with uClibc.

According to uClibc commit [1], dlinfo is not available.
To be able to use LTTng UST with uClibc, we need to disable
the Dynamic Linker Tracing functionality [2].

Fixes:
http://autobuild.buildroot.net/results/a6c/a6c33dd7ec2a36a50c5ea74b989a371d6c85e899/build-end.log

[1] http://git.uclibc.org/uClibc/commit/?id=f3c9dc499c5c787ddd8c4320f2d44d2ae6e40c22
[2] http://lists.lttng.org/pipermail/lttng-dev/2014-February/022423.html



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>