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

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>