Fix find/rm command in Unix clean recipe

Fix find/rm command in Unix clean recipe

The `./pyca-cryptography/.travis/downstream.d` subdirectory that causes the `rm` command to fail (albeit harmlessly, but with a warning from `make` nonetheless).

>rm -f `find . -name '*.d' \! -name '.*' -print`
>rm: cannot remove './pyca-cryptography/.travis/downstream.d': Is a directory
>make: [Makefile:1910: clean] Error 1 (ignored)

Exclude directories from being matched by the `find` commands.

CLA: trivial

Reviewed-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10264)