tracing/function-graph-tracer: strip ending newlines on comments

Impact: tracer output improvement

Ending newlines are appended automatically on comments by the function
graph tracer because the newline needs to be placed after the "*/"
comment characters.

So if the user puts an ending newline, we want to strip it.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>