scripts: Added missing sys imports

Unfortunately the import sys in the argparse block was hiding missing
sys imports.

The mistake was assuming the import sys in Python would limit the scope
to that if block, but Python's late binding strikes again...

scripts/perfbd.py

@@ -26,6 +26,7 @@ import os
 import re
 import shlex
 import subprocess as sp
+import sys
 
 
 OBJDUMP_PATH = ['objdump']