initramfs-build: Add rsync to container

Change-Id: I5f0536b4d4cfc32ebd345dd8658bf6581334ea6f
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/initramfs-build.sh b/initramfs-build.sh
index 4ae3d9f..590d94c 100755
--- a/initramfs-build.sh
+++ b/initramfs-build.sh
@@ -80,6 +80,7 @@
 	python \
 	unzip \
 	wget \
+	rsync \
 	iputils-ping \
 	locales