Add .gitignore for Arch Linux Packages

Mandatory files for a Arch Linux package are

* PKGBUILD
* install scripts, if any
* user defined configs/patches, if any

This .gitignore ignores files that are
generated/downloaded by makepkg automatically.

Refs:

* https://wiki.archlinux.org/index.php/Creating_Packages
* https://wiki.archlinux.org/index.php/Makepkg

Signed-off-by: Lance Chen <cyen0312@gmail.com>