travis.yml: Install libcap-dev for testing virito-9p

So far we were not testing virtio-9p in Travis yet, since we forgot to
install libcap-devel. Do it now to get some more test coverage.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Acked-by: NGreg Kurz <groug@kaod.org>
Tested-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190905113346.2473-1-thuth@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -26,6 +26,7 @@ addons:
       - libaio-dev
       - libattr1-dev
       - libbrlapi-dev
+      - libcap-dev
       - libcap-ng-dev
       - libgcc-4.8-dev
       - libgnutls-dev