update make clobber to remove example files in 1987/heckbert

1987/heckbert/Makefile

@@ -226,6 +226,7 @@ clean:
 
 clobber: clean
 	${RM} -f ${TARGET} ${ALT_TARGET}
+	${RM} -f bar.c ph ph.c
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \
 	    echo 'ate sandwich'; \