cipher: Add Classic McEliece mceliece6688128f.
* cipher/Makefile.am (libcipher_la_SOURCES): Add mceliece6688128f.{c,h}.
* cipher/mceliece6688128f.c, cipher/mceliece6688128f.h: New.
* tests/t-kem.c: Support mceliece6688128f.- Signed-off-by: Simon Josefsson <simon@josefsson.org>