[bazel] Add missing `includes` path.

Add `includes = ["."]` to the `BUILD.bazel` file. This is
necessary to allow dependents to compile with a simple
`#include <zlib.h>` line, like libcurl (and others) do.

Testing: fx build --host @//third_party/curl:libcurl
Change-Id: I6bb48338b34d4f31e5e08fe2b18478605885eaaf
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/zlib/+/1779074
Reviewed-by: Francois Rousseau <frousseau@google.com>