add clean to csf-supplementary/Makefile

paper/csf-supplementary/Makefile

 previous.pdf: previous.tex usenix-*.tex tweetverif-USENIX.pdf tweetverif-SP.pdf
 	pdflatex previous.tex
 
+clean:
+	-rm previous.aux
+	-rm previous.log
+	-rm previous.out
+	-rm previous.pdf
+	-rm previous.thm
+	-rm previous.bbl
+	-rm previous.blg
+	-rm previous.brf