README: Add gcc-multilib to dependencies

gcc-multilib is required on host for compiling luajit.
If it is not installed, luajit fails with this error:

fatal error: bits/libc-header-start.h: No such file or directory

We use Luajit for Knot Resolver, Suricata.

README.asciidoc

@@ -29,7 +29,7 @@ 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
+  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python file rsync gcc-multilib
 --
 
 For other distributions, we suggest to look at https://openwrt.org/docs/guide-developer/build-system/install-buildsystem[OpenWrt documentation].