make: fix build.

The .gitignore file hid the build-aux and gnulib directories that were
needed to successfully build locally, breaking the NDK build (but not
the soong build).

I'm not sure why presubmit didn't catch this.

Tested locally with `mv toolchain/make /tmp && repo sync toolchain/make`
and then restoring files until checkbuild.py succeeded.

Bug: http://b/153310680
Test: checkbuild.py --module make
Change-Id: I9602060e61fcfb01df6a4ae2d50b27bd78592b15