.gitignore: add sys/*/compile

Files in sys/*/compile are created when compiling the kernel with
config(8). They are never source files and should never be committed to
source control, so list this entire directory in .gitignore. While not
the official way to build the kernel, it's often useful to debug
sys/conf/files* changes when adding new drivers, etc.

Reviewed by: imp, meena <me igalic co>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1082