Avoid pointer arithmetic on "void *"

Fix the following warning (with -Wpedantic enabled):

    pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]

app/src/net.c

@@ -94,7 +94,7 @@ ssize_t net_send_all(socket_t socket, const void *buf, size_t len) {
             return -1;
         }
         len -= w;
-        buf += w;
+        buf = (char *) buf + w;
     }
     return w;
 }