travis: install graphviz in doxygen test

Install graphviz to stop doxygen from emitting 'missing dot' errors.

Signed-off-by: Dmitry Eremin-Solenikov <dmitry.ereminsolenikov@linaro.org>
Signed-off-by: Balasubramanian Manoharan <bala.manoharan@linaro.org>
Reviewed-by: Bill Fischofer <bill.fischofer@linaro.org>
Reviewed-by: Petri Savolainen <petri.savolainen@linaro.org>
Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>

.travis.yml

@@ -183,6 +183,7 @@ jobs:
                                           - libconfig-dev
                                           - libssl-dev
                                           - cmake
+                                          - graphviz
                   install:
                           # Updated Doxygen
                         - |