qemu: Add ca-certificates to container

Qemu builds are failing due to git's inability to verify a certificate:

fatal: unable to access 'https://git.qemu.org/git/capstone.git/':
Problem with the SSL CA cert (path? access rights?)

This is because the system does not have the CA certificate package
installed.

Change-Id: Ibfff0927dd551eed38901efa4400e7e5b328595f
Signed-off-by: Joel Stanley <joel@voyager.ozlabs.ibm.com>
diff --git a/qemu-build.sh b/qemu-build.sh
index 9468159..590a535 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -107,6 +107,7 @@
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && apt-get install -yy --no-install-recommends \
     bison \
+    ca-certificates \
     flex \
     gcc \
     git \