kernel-build: Add missing deps

The kernel now requires flex and bison.

Change-Id: Ie45c48665c127a87ef8cbbc5851ac8f83d47fe7c
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/kernel-build.sh b/kernel-build.sh
index 24ee3ab..d736a90 100755
--- a/kernel-build.sh
+++ b/kernel-build.sh
@@ -31,6 +31,8 @@
 	gcc-powerpc64le-linux-gnu \
 	software-properties-common \
 	libssl-dev \
+	bison \
+	flex \
 	iputils-ping
 
 RUN apt-add-repository -y multiverse && apt-get update && apt-get install -yy \