trace: Do not hide operator() functions

They are important in some cases so we should not hide them. Lambda
functions are reported like that.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: update unimportant_prefixes

The version built on muninn does not have the leading
'tracepoint_base::log_backtrace(trace_record*, unsigned char*&)'
frame.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>