Clear the exit code from 'find' in 'make depend'

Depending on what has been built so far, all .d files may not be
present and 'find' will exit with non-zero exit code.  This isn't a
bother for us but may break make, so clear the exit code with an added
'exit 0'.

Closes RT#4444

Reviewed-by: Rich Salz <rsalz@openssl.org>