perf scripts python: Fix passing arguments to stackcollapse report

The '--' prevented arguments from being passed to the script, such as:

  $ perf script report stackcollapse -i my_perf.data

Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
LPU-Reference: 20200427142327.21172-1-mpetlan@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>