.gitignore: add autotools' `compile`

Since automake 1.14, autotools now generate a symbolic link named
`compile` in the root of the repository when running, so let's include
it in .gitignore to achieve a clean `git status` after compiling (or
avoid breaking the build via `git clean -fd`).

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>