lttng-modules: fix build on linux 3.14.x

By applying patch from upstream mailing list[1] (should be in the next
release).

[1]: http://lists.lttng.org/pipermail/lttng-dev/2014-December/023914.html

(cherry picked from commit 507e78fb94376040e74588dd7a31eb44b32d1e91)