README: add libc6-dev-i386 dependency

This dependency is required while compiling moonjit (it's luajit's
fork). Moonjit is used, for example, by Knot Resolver.
Otherwise, compilation fails with the following output:

/usr/include/gnu/stubs.h:7:11: fatal error: gnu/stubs-32.h: No such file or directory
    7 | # include <gnu/stubs-32.h>
      |           ^~~~~~~~~~~~~~~~
compilation terminated.

Reported-by: Michal Vasilek <michal.vasilek@nic.cz>

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 python3 file rsync gcc-multilib g++-multilib
+  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python python3 file rsync gcc-multilib g++-multilib libc6-dev-i386
 --
 
 For other distributions, we suggest to look at https://openwrt.org/docs/guide-developer/build-system/install-buildsystem[OpenWrt documentation].