HashML-DSA: Replace enum by #define

[mkannwischer]

Currently we are using an enum to specify which pre-hashing hash function to be used:

mldsa-native/mldsa/sign.h

Lines 495 to 499 in 1d7ed8d

	int crypto_sign_verify_pre_hash_internal(const uint8_t *sig, size_t siglen,
	const uint8_t *ph, size_t phlen,
	const uint8_t *ctx, size_t ctxlen,
	const uint8_t *pk,
	mld_hash_alg_t hashAlg)

mldsa-native/mldsa/sign.h

Lines 48 to 62 in 1d7ed8d

	typedef enum
	{
	MLD_SHA2_224,
	MLD_SHA2_256,
	MLD_SHA2_384,
	MLD_SHA2_512,
	MLD_SHA2_512_224,
	MLD_SHA2_512_256,
	MLD_SHA3_224,
	MLD_SHA3_256,
	MLD_SHA3_384,
	MLD_SHA3_512,
	MLD_SHAKE_128,
	MLD_SHAKE_256
	} mld_hash_alg_t;

This leads to problems when introducing a separate mldsa_native.h specifying the public API that can be included multiple times as it introduces clashes. #537 works around this guarding it by an #ifndef to only define it once.
Futhermore, there is a (theoretical) concern about the type of the enum being implementation-defined in C90: #537 (comment).

We should eliminate the enum and instead use #defines to define numerical values for each possible value.