Documentation: Add texinfo to build instructions

`Meta/serenity.sh rebuild-toolchain aarch64` failed with the gdb build
complaining about missing `makeinfo` without this.

Documentation/BuildInstructions.md

@@ -7,7 +7,7 @@ Make sure you have all the dependencies installed:
 ### Debian / Ubuntu
 
 ```console
-sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-i386 qemu-utils ccache rsync genext2fs unzip
+sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-i386 qemu-utils ccache rsync genext2fs unzip texinfo
 ```
 
 #### GCC 10