.travis.yml: add libnfs-dev for NFS block driver

Let's ensure that block/nfs.o is built in Travis.

This patch depends on the following build fixes:
1. block/nfs: add missing #include "qapi/error.h"
2. block/nfs: add missing #include "qemu/cutils.h"

This patch also depends on Travis adding libnfs-dev to the list of
approved packages.  This patch can be safely committed but will not do
anything until the Travis maintainers allow libnfs-dev to be installed.
Please see the GitHub Issue I raised here:
travis-ci/apt-package-safelist#2788

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -17,6 +17,7 @@ addons:
       - libgtk-3-dev
       - libiscsi-dev
       - liblttng-ust-dev
+      - libnfs-dev
       - libncurses5-dev
       - libnss3-dev
       - libpixman-1-dev