Travis: move doxygen dependencies into the "sanity" job

Only this job uses doxygen and graphviz.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -6,6 +6,11 @@ cache: ccache
 jobs:
   include:
     - name: basic checks
+      addons:
+        apt:
+          packages:
+          - doxygen
+          - graphviz
       script:
         - tests/scripts/recursion.pl library/*.c
         - tests/scripts/check-generated-files.sh
@@ -38,10 +43,6 @@ env:
     - secure: "FrI5d2s+ckckC17T66c8jm2jV6i2DkBPU5nyWzwbedjmEBeocREfQLd/x8yKpPzLDz7ghOvr+/GQvsPPn0dVkGlNzm3Q+hGHc/ujnASuUtGrcuMM+0ALnJ3k4rFr9xEvjJeWb4SmhJO5UCAZYvTItW4k7+bj9L+R6lt3TzQbXzg="
 
 addons:
-  apt:
-    packages:
-    - doxygen
-    - graphviz
   coverity_scan:
     project:
       name: "ARMmbed/mbedtls"