.gitignore: Add Kdevelop4 project files

I'm not sure what is the costume with such IDE project files.
Most might be dot-files. It is kind of annoying for the Kdevelop4 user.

So please consider adding *.kdev4 files to be ignored?

Signed-off-by: Boaz Harrosh <boaz@plexistor.com>
[mmarek: Moved at the and and added a comment]
Signed-off-by: Michal Marek <mmarek@suse.cz>