only compile yara rules the first time they are used
only compile yara rules the first time they are used

This saves about 1.5 second per 1000 times a file is checked against the
default Yara signatures in Cuckoo. We can assume that Cuckoo or
process.py is restarted after Yara rules are updated - in the end, the
same goes for updating Signatures.

cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo