lttng-tools: fix build with gcc 5.x

This commit backports an upstream commit from lttng-tools to solve gcc
5.x related build issues.

Fixes:

  http://autobuild.buildroot.org/results/65e/65e0fbddfee14aff84a4ca59cd7315e1e250b55b/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>