tools: add missing exec bit to softirqslower.py

The smoke tool test runs tool directly, and especially softirqslower.py
lacked the exec bit and failed with "Permission denied". Set the mode
to 0755 so the script can be executed by the tests.

    29: ....
    timeout: failed to run command ‘/bcc/tools/softirqslower.py’: Permission denied

Signed-off-by: Hoyeon Lee <[email protected]>

Author: HoyeonRhee