package/privoxy: security bump to version 3.0.31

package/privoxy: security bump to version 3.0.31

From the announcement:

ChangeLog for Privoxy 3.0.31
--------------------------------------------------------------------
- Security/Reliability:
  - Prevent an assertion from getting triggered by a crafted CGI request.
    Commit 5bba5b89193fa. OVE-20210130-0001.
    Reported by: Joshua Rogers (Opera)
  - Fixed a memory leak when decompression fails "unexpectedly".
    Commit f431d61740cc0. OVE-20210128-0001.

- Bug fixes:
  - Fixed detection of insufficient data for decompression.
    Previously Privoxy could try to decompress a partly
    uninitialized buffer.

https://www.privoxy.org/announce.txt

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>