Fix a make error when there is no *.pyc files.
Fix a make error when there is no *.pyc files.

When there is no *.pyc to delete, GNU xargs will still run the command once
with no arguments, which causes an error, stopping make. The -r flag prevents
this.

RJPercival/certificate-transparency,eranmes/certificate-transparency,pphaneuf/certificate-transparency,AlCutter/certificate-transparency,aeijdenberg/certificate-transparency,benlaurie/certificate-transparency,lexibrent/certificate-transparency,phad/certificate-transparency,grandamp/certificate-transparency,grandamp/certificate-transparency,rep/certificate-transparency,ksmaheshkumar/certificate-transparency,aeijdenberg/certificate-transparency,ksmaheshkumar/certificate-transparency,kyprizel/certificate-transparency,benlaurie/certificate-transparency,ksmaheshkumar/certificate-transparency,eranmes/certificate-transparency,AlCutter/certificate-transparency,benlaurie/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,grandamp/certificate-transparency,plietar/certificate-transparency,aeijdenberg/certificate-transparency,Martin2112/certificate-transparency,plietar/certificate-transparency,phad/certificate-transparency,katjoyce/certificate-transparency,katjoyce/certificate-transparency,plietar/certificate-transparency,eranmes/certificate-transparency,RJPercival/certificate-transparency,katjoyce/certificate-transparency,lexibrent/certificate-transparency,ksmaheshkumar/certificate-transparency,Martin2112/certificate-transparency,RJPercival/certificate-transparency,kyprizel/certificate-transparency,google/certificate-transparency,Martin2112/certificate-transparency,eranmes/certificate-transparency,rep/certificate-transparency,plietar/certificate-transparency,kyprizel/certificate-transparency,aeijdenberg/certificate-transparency,katjoyce/certificate-transparency,rep/certificate-transparency,rep/certificate-transparency,Martin2112/certificate-transparency,phad/certificate-transparency,google/certificate-transparency,aeijdenberg/certificate-transparency,lexibrent/certificate-transparency,Martin2112/certificate-transparency,pphaneuf/certificate-transparency,AlCutter/certificate-transparency,taknira/certificate-transparency,rep/certificate-transparency,phad/certificate-transparency,benlaurie/certificate-transparency,pphaneuf/certificate-transparency,ksmaheshkumar/certificate-transparency,eranmes/certificate-transparency,taknira/certificate-transparency,ksmaheshkumar/certificate-transparency,AlCutter/certificate-transparency,kyprizel/certificate-transparency,google/certificate-transparency,kyprizel/certificate-transparency,katjoyce/certificate-transparency,taknira/certificate-transparency,aeijdenberg/certificate-transparency,aeijdenberg/certificate-transparency,taknira/certificate-transparency,Martin2112/certificate-transparency,katjoyce/certificate-transparency,katjoyce/certificate-transparency,taknira/certificate-transparency,taknira/certificate-transparency,plietar/certificate-transparency,kyprizel/certificate-transparency,eranmes/certificate-transparency,benlaurie/certificate-transparency,rep/certificate-transparency,ksmaheshkumar/certificate-transparency,plietar/certificate-transparency,AlCutter/certificate-transparency,grandamp/certificate-transparency,benlaurie/certificate-transparency,kyprizel/certificate-transparency,rep/certificate-transparency,AlCutter/certificate-transparency,pphaneuf/certificate-transparency,Martin2112/certificate-transparency,taknira/certificate-transparency,plietar/certificate-transparency,lexibrent/certificate-transparency,benlaurie/certificate-transparency,eranmes/certificate-transparency,AlCutter/certificate-transparency