liblttng-ust-libc: fix lib dependency

liblttng-ust-libc: fix lib dependency

liblttng-ust-libc.so depends on liblttng-ust.so but does not link
against it.

z$ ../linux32-cross-i686-aldebaran-linux-gnu/bin/i686-aldebaran-linux-gnu-nm -CD output/target/usr/lib/liblttng-ust-libc.so | grep -E 'U ltt'
         U ltt_probe_register
         U ltt_probe_unregister
z$ ../linux32-cross-i686-aldebaran-linux-gnu/bin/i686-aldebaran-linux-gnu-ldd --root output/target/ output/target/usr/lib/liblttng-ust-libc.so
        liburcu-bp.so.1 => /usr/lib/liburcu-bp.so.1 (0xdeadbeef)
        libpthread.so.0 => /lib/libpthread.so.0 (0xdeadbeef)
        libc.so.6 => /lib/libc.so.6 (0xdeadbeef)
        ld-linux.so.2 => /lib/ld-linux.so.2 (0xdeadbeef)
        liburcu-common.so.1 => /usr/lib/liburcu-common.so.1 (0xdeadbeef)
        libuuid.so.1 => /lib/libuuid.so.1 (0xdeadbeef)
        libdl.so.2 => /lib/libdl.so.2 (0xdeadbeef)
z$ ../linux32-cross-i686-aldebaran-linux-gnu/bin/i686-aldebaran-linux-gnu-nm -CD output/target/usr/lib/liblttng-ust.so | grep ltt_probe_
0000a5cc T ltt_probe_register
0000a7d6 T ltt_probe_unregister
z$

Signed-off-by: Samuel Martin <smartin@aldebaran-robotics.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>