gitignore: Add ffi_lib.so to the gitignore list.

When testing the Unix port with FFI tests enabled, the FFI helper
library that's built as part of the testing process is not removed after
the tests session end.

This commit adds the built library to the gitignore list, so it doesn't
mark the sourcetree as dirty if a test run for the Unix port was started
in the past.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

.gitignore

@@ -11,8 +11,9 @@ build/
 build-*/
 docs/genrst/
 
-# Test failure outputs
+# Test failure outputs and intermediate artefacts
 tests/results/*
+tests/ports/unix/ffi_lib.so
 
 # Python cache files
 __pycache__/