UBUNTU: SAUCE: (no-up) fix build error with CONFIG_BLK_DEV_INITRD=n

The original aynchronous boot patch (UBUNTU: SAUCE: Make populate_rootfs asynchronous)
did not take into consideration the case when CONFIG_BLK_DEV_INITRD=n,
e.g., populate_rootfs_domain becomes undefined. Therefore, add it to
noinitramfs.c where its use is benign.
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Original-patch-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Surbhi Palande <surbhi.palande@canonical.com>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>

init/noinitramfs.c

@@ -22,6 +22,8 @@
 #include <linux/kdev_t.h>
 #include <linux/syscalls.h>
 
+LIST_HEAD(populate_rootfs_domain);
+
 /*
  * Create a simple rootfs that is similar to the default initramfs
  */