libvncclient: cast buf when calling GotXCutTextUTF8()

buf is an unsigned char pointer while GotXCutTextUTF8() takes a char
pointer.

Author: tobydox

src/libvncclient/rfbclient.c

@@ -1967,7 +1967,7 @@ rfbClientProcessExtServerCutText(rfbClient* client, char *data, int len)
     return FALSE;
   }
   if (client->GotXCutTextUTF8)
-    client->GotXCutTextUTF8(client, buf, size);
+    client->GotXCutTextUTF8(client, (char *)buf, size);
   free(buf);
 
   inflateEnd(&stream);