[mod] limiter: block requests from PetalBot

Block requests from PetalBlock.  Normally robots.txt is enough to stop
PetalBlock from making requests [1].  However, if SearXNG is offered below a
path (example.org/search), then the robots.txt is not available in the root
paths of the domain / subdomain.

[1] https://webmaster.petalsearch.com/site/petalbot



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>