lttng-modules: Check for v4l instrumentation in kernel

Checks availability of trace header file at include/trace/events
in kernel source tree before trying to build lttng v4l probes.
Previously build was breaking while building v4l probes for BSPs
where kernel wasn't instrumented with v4l traces.

Signed-off-by: Yasir-Khan <yasir_khan@mentor.com>
Merge pull request #16 from Yasir-Khan/v4l_check

lttng-modules: Check for v4l instrumentation in kernel