makefile: Add a cscope target

makefile: Add a cscope target

The current makefile supports ctags but not cscope. Some people prefer
cscope (I do), so this patch adds a cscope target.

I've also added cscope* to the .gitignore file. For some reason tags
and TAGS weren't in there either so I've added them too.

Signed-off-by: Kristof Provost <Kristof@provost-engineering.be>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>