Add generated cipher implementation files to gitignore

Add generated cipher implementation files to gitignore

A commit was merged recently which change ciphercommon.c and
cipher_chacha20_poly1305.c to be generated files. The corresponding
.c files were not added to the gitignore, so this commit adds them
to avoid them being erroneously tracked in the future.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27715)