Make secp256k1_ecdh_{,xonly}_hash_function work on MSVC

Author: sipa

src/modules/ecdh/main_impl.h

@@ -10,7 +10,7 @@
 #include "../../../include/secp256k1_ecdh.h"
 #include "../../ecmult_const_impl.h"
 
-static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char *x32, const unsigned char *y32, void *data) {
+int secp256k1_ecdh_hash_function_sha256_internal(unsigned char *output, const unsigned char *x32, const unsigned char *y32, void *data) {
     unsigned char version = (y32[31] & 0x01) | 0x02;
     secp256k1_sha256 sha;
     (void)data;
@@ -23,7 +23,7 @@ static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char
     return 1;
 }
 
-static int ecdh_xonly_hash_function_sha256(unsigned char *output, const unsigned char *x32, void *data) {
+int secp256k1_ecdh_xonly_hash_function_sha256_internal(unsigned char *output, const unsigned char *x32, void *data) {
     secp256k1_sha256 sha;
     (void)data;
 
@@ -34,9 +34,9 @@ static int ecdh_xonly_hash_function_sha256(unsigned char *output, const unsigned
     return 1;
 }
 
-const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256 = ecdh_hash_function_sha256;
-const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default = ecdh_hash_function_sha256;
-const secp256k1_ecdh_xonly_hash_function secp256k1_ecdh_xonly_hash_function_sha256 = ecdh_xonly_hash_function_sha256;
+const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256 = &secp256k1_ecdh_hash_function_sha256_internal;
+const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default = &secp256k1_ecdh_hash_function_sha256_internal;
+const secp256k1_ecdh_xonly_hash_function secp256k1_ecdh_xonly_hash_function_sha256 = &secp256k1_ecdh_xonly_hash_function_sha256_internal;
 
 int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const secp256k1_pubkey *point, const unsigned char *scalar, secp256k1_ecdh_hash_function hashfp, void *data) {
     int ret = 0;