Add stdio header to hello.c

Signed-off-by: oooxff <oooxff@gmail.com>
Add stdlib header to hello.c

Signed-off-by: oooxff <oooxff@gmail.com>
Use sum function.

Signed-off-by: oooxff <oooxff@gmail.com>
Remove unnecessary tab space.

Signed-off-by: oooxff <oooxff@gmail.com>