packaging: Redirect stderr to /dev/null in %post action (frr.spec.in)

packaging: Redirect stderr to /dev/null in %post action (frr.spec.in)

Solves:
```
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
/usr/bin/sed: couldn't write 40 items to stdout: Broken pipe
```

This happens because `grep -q` returns immediately after first match
and closes the pipe while sed has more output to write.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>