lttng-tools: Enable aarch64 build

Now that liburcu builds for aarch64, lttng-tools has all its dependencies met
for aarch64, so allow it to build for this architecture.

Signed-off-by: Ben Shelton <benjamin.h.shelton@intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>