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

(cherry picked from commit 38b71bd4704ee1746e862f5a7a4e170fd84a5eb0)

Configurations/unix-Makefile.tmpl

@@ -516,8 +516,8 @@ libclean:
 clean: libclean
 	$(RM) $(PROGRAMS) $(TESTPROGS) $(ENGINES) $(SCRIPTS)
 	$(RM) $(GENERATED_MANDATORY) $(GENERATED)
-	-$(RM) `find . -name '*{- $depext -}' \! -name '.*' -print`
-	-$(RM) `find . -name '*{- $objext -}' \! -name '.*' -print`
+	-$(RM) `find . -name '*{- $depext -}' \! -name '.*' \! -type d -print`
+	-$(RM) `find . -name '*{- $objext -}' \! -name '.*' \! -type d -print`
 	$(RM) core
 	$(RM) tags TAGS doc-nits
 	$(RM) -r test/test-runs