fix genfilelist

Fixed the sort for `make genfilelist` to ignore case
and to use the ${SORT} makefile variable.

2019/.filelist

@@ -1,11 +1,11 @@
 2019/2019.tar.bz2
-2019/Makefile
-2019/README.md
 2019/.filelist
 2019/.gitignore
 2019/guidelines.txt
 2019/index.html
 2019/iocccsize.c
 2019/iocccsize.mk
+2019/Makefile
+2019/README.md
 2019/rules.txt
 2019/.year