perf script python: Fix export-to-sqlite.py sample columns

perf script python: Fix export-to-sqlite.py sample columns

With the "branches" export option, not all sample columns are exported.
However the unwanted columns are not at the end of the tuple, as assumed
by the code. Fix by taking the first 15 and last 3 values, instead of
the first 18.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20180911114504.28516-3-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>