Add gmock to package.xml

Author: dsobek

package.xml

@@ -23,7 +23,7 @@
   <depend>libsqlite3-dev</depend>
   <depend>libzmq3-dev</depend>
 
-  <test_depend >ament_cmake_gtest</test_depend>
+  <test_depend >ament_cmake_gmock</test_depend>
 
   <export>
       <build_type >ament_cmake</build_type>