Add warning for perf_event_paranoid >= 3

[topolarity]

Paranoia settings this high don't allow even user-level perf monitoring of your own process.

That documentation can be hard to find online so this adds a warning to nudge the user in the right direction:

julia> bench = make_bench([LinuxPerf.EventType(:hw, :instructions)])
┌ Warning: try to adjust /proc/sys/kernel/perf_event_paranoid to a value <= 2 or run with the CAP_PERFMON capability
└ @ LinuxPerf ~/.julia/dev/LinuxPerf/src/LinuxPerf.jl:301
ERROR: perf_event_open error : Permission denied