Update build dependencies

Fixes microsoft/WSL#11738

Signed-off-by: Mitchell Levy <mitchelllevy@microsoft.com>

Author: chessturo

README.md

@@ -28,7 +28,7 @@ Instructions for building an x86_64 WSL2 kernel with an Ubuntu distribution are
 as follows:
 
 1. Install the build dependencies:  
-   `$ sudo apt install build-essential flex bison dwarves libssl-dev libelf-dev`
+   `$ sudo apt install build-essential flex bison dwarves libssl-dev libelf-dev cpio`
 
 2. Modify WSL2 kernel configs (optional):  
    `$ make menuconfig KCONFIG_CONFIG=Microsoft/config-wsl`