darwinssl: fix curlssl_sha256sum() compiler warnings on first argument

lib/vtls/darwinssl.c

@@ -2722,9 +2722,9 @@ void Curl_darwinssl_md5sum(unsigned char *tmp, /* input */
 }
 
 void Curl_darwinssl_sha256sum(unsigned char *tmp, /* input */
-                           size_t tmplen,
+                              size_t tmplen,
-                           unsigned char *sha256sum, /* output */
+                              unsigned char *sha256sum, /* output */
-                           size_t sha256len)
+                              size_t sha256len)
 {
   assert(sha256len >= SHA256_DIGEST_LENGTH);
   (void)CC_SHA256(tmp, (CC_LONG)tmplen, sha256sum);

lib/vtls/darwinssl.h

@@ -49,9 +49,9 @@ void Curl_darwinssl_md5sum(unsigned char *tmp, /* input */
                            unsigned char *md5sum, /* output */
                            size_t md5len);
 void Curl_darwinssl_sha256sum(unsigned char *tmp, /* input */
-                           size_t tmplen,
+                              size_t tmplen,
-                           unsigned char *sha256sum, /* output */
+                              unsigned char *sha256sum, /* output */
-                           size_t sha256len);
+                              size_t sha256len);
 bool Curl_darwinssl_false_start(void);
 
 /* Set the API backend definition to SecureTransport */
@@ -93,7 +93,8 @@ bool Curl_darwinssl_false_start(void);
 #define curlssl_data_pending(x,y) Curl_darwinssl_data_pending(x, y)
 #define curlssl_random(x,y,z) ((void)x, Curl_darwinssl_random(y,z))
 #define curlssl_md5sum(a,b,c,d) Curl_darwinssl_md5sum(a,b,c,d)
-#define curlssl_sha256sum(a,b,c,d) Curl_darwinssl_sha256sum(a,b,c,d)
+#define curlssl_sha256sum(a,b,c,d) \
+  Curl_darwinssl_sha256sum((unsigned char *)a, b, c, d)
 #define curlssl_false_start() Curl_darwinssl_false_start()
 
 #endif /* USE_DARWINSSL */