trivial: support cscope in Makefile and gitignore

After applying this patch, "make cscope" can generate cscope files for
tag jump. The generated files are ignored by the .gitignore file.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>