cipher:kem: Provide each enum constant as macro.

src/gcrypt.h.in (GCRY_KEM_CM6688128F, GCRY_KEM_DHKEM25519)
(GCRY_KEM_DHKEM448, GCRY_KEM_DHKEMP256R1, GCRY_KEM_DHKEMP384R1)
(GCRY_KEM_DHKEMP521R1, GCRY_KEM_RAW_BP256, GCRY_KEM_RAW_BP384)
(GCRY_KEM_RAW_BP512, GCRY_KEM_RAW_MLKEM512, GCRY_KEM_RAW_MLKEM768)
(GCRY_KEM_RAW_MLKEM1024, GCRY_KEM_RAW_P256K1, GCRY_KEM_RAW_P256R1)
(GCRY_KEM_RAW_P384R1, GCRY_KEM_RAW_P521R1, GCRY_KEM_RAW_X25519)
(GCRY_KEM_RAW_X448, GCRY_KEM_SNTRUP761): New defines.

• GnuPG-bug-id: T7698
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>