[pcie-check] Make pcie-check.sh executable (#5256)

The pcie-check.sh script was added in #4771, but was not given executable permission. Therefore, we would see messages like:

```
Aug 26 22:54:05.536248 sonic ERR systemd[664]: pcie-check.service: Failed to execute command: Permission denied
Aug 26 22:54:05.536386 sonic ERR systemd[664]: pcie-check.service: Failed at step EXEC spawning /usr/bin/pcie-check.sh: Permission denied
Aug 26 22:54:05.536600 sonic WARNING systemd[1]: pcie-check.service: Failed with result 'exit-code'.
```