Add Makefile rule to extract puzzle solutions

Makefile

@@ -41,6 +41,9 @@ solve.o: solve.h
 puzzles: sudoku.csv
 	pv sudoku.csv | tail -n+2 | cut -d, -f1 | tr -d '\n' > $@
 
+solns: sudoku.csv
+	pv sudoku.csv | tail -n+2 | cut -d, -f2 | tr -d '\n' > $@
+
 results:
 	dd if=/dev/zero of=$@ bs=1000 count=729000