Update .gitignore to track changes to esp32/components/avm_builtins.

[UncleGrumpy]

Changes to the contents of AtomVM/src/platforms/esp32/components/avm_builtins were ignored by the gitignore rules. This updates the rules to track changes to the avm_builtins directory.

Closes issue #330.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later