deps: add cmocka as submodule for unit testing

.travis.yml

@@ -40,6 +40,7 @@ before_install:
   - pip install --user cpp-coveralls
 
 script:
+  - sh ./build-deps.sh
   - sh ./autogen.sh
   - if [[ "$CC" == "gcc" && "$CODE_COVERAGE" == "ON" ]]; then
       ./configure CFLAGS="-ftest-coverage -fprofile-arcs";