perf scripts python: exported-sql-viewer.py: Fix missing shebang

exported-sql-viewer.py is a standalone python script and requires a
shebang. Also only python2 is supported at present. Restore the shebang
but use the more flexible 'env' form.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: stable@vger.kernel.org
Fixes: a38352de

 ("perf script python: Remove explicit shebang from Python script")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>