Rename the decaf files to curve448 files

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5105)

diff --git a/crypto/ec/curve448/decaf.c b/crypto/ec/curve448/curve448.c
similarity index 100%
rename from crypto/ec/curve448/decaf.c
rename to crypto/ec/curve448/curve448.c

diff --git a/crypto/ec/curve448/decaf_tables.c b/crypto/ec/curve448/curve448_tables.c
similarity index 100%
rename from crypto/ec/curve448/decaf_tables.c
rename to crypto/ec/curve448/curve448_tables.c