darwinssl: fix curlssl_sha256sum() compiler warnings on first argument

lib/vtls/darwinssl.h

@@ -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 */