tracing: Remove unused variable in trace_benchmark

Somehow this unused variable warning sneaked past my warnings check
(probably due to it depending on a new config).

kernel/trace/trace_benchmark.c: In function 'trace_do_benchmark':
kernel/trace/trace_benchmark.c:38:6: warning: unused variable 'seedsq' [-Wunused-variable]
  u64 seedsq;
      ^

Link: http://lkml.kernel.org/r/20140604160921.4f4e69c4@canb.auug.org.au

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>