gitignore: ignore all cscope generated files

gitignore: ignore all cscope generated files

When using cscope to read code, it will generate below 3 files for speedup
cross-refer: cscope.files, cscope.in.out, cscope.po.out

The .gitignore only contains "/cscope.out". It a little bit messy when
executing 'git status', and other git commands.
This patch add all cscope generated files in .gitignore.

Signed-off-by: Zhao Heming <heming.zhao@suse.com>