kernel-build: Add libssl to container

Some modern kernel configurations require libssl.

Change-Id: I0c15fccab13607f4bd31a50a148ad7c896491474
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/kernel-build.sh b/kernel-build.sh
index 83f0d56..cd91e68 100755
--- a/kernel-build.sh
+++ b/kernel-build.sh
@@ -30,6 +30,7 @@
 	git \
 	gcc-powerpc64le-linux-gnu \
 	software-properties-common \
+	libssl-dev \
 	iputils-ping
 
 RUN apt-add-repository -y multiverse && apt-get update && apt-get install -yy \