README: add g++-multilib as requirement

While building node packages on Ubuntu 20.04 LTS, node fails because of:

In file included from /usr/include/c++/9/deque:60,
                 from /usr/include/c++/9/queue:60,
                 from ../deps/v8/src/libplatform/default-foreground-task-runner.h:8,
                 from ../deps/v8/src/libplatform/default-foreground-task-runner.cc:5:
/usr/include/c++/9/bits/stl_algobase.h:59:10: fatal error: bits/c++config.h: No such file or directory
   59 | #include <bits/c++config.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

This can be solved by installing g++-multilib

README.adoc

@@ -36,7 +36,7 @@ You need to have a GNU/Linux distribution to be able to compile Turris OS and pa
 To install all dependencies for Debian distribution you can use:
 
 --
-  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python file rsync gcc-multilib
+  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python file rsync gcc-multilib g++-multilib
 --
 
 For other distributions, we suggest to look at https://openwrt.org/docs/guide-developer/build-system/install-buildsystem[OpenWrt documentation].