Fix rosidl_generator_java_warnings (#77)

Signed-off-by: Ivan Santiago Paunovic <ivanpauno@ekumenlabs.com>

Author: ivanpauno

rosidl_generator_java/CMakeLists.txt

@@ -40,7 +40,7 @@ if(BUILD_TESTING)
 
   # TODO(jacobperron): This fixes generating messages with wstrings.
   # Figure out why this is needed as of Foxy, and how we can remove this workaround
-  set(CMAKE_JAVA_COMPILE_FLAGS "-encoding" "ISO-8859-1")
+  list(APPEND CMAKE_JAVA_COMPILE_FLAGS "-encoding" "ISO-8859-1")
 
   include(cmake/register_java.cmake)
   include(cmake/rosidl_generator_java_get_typesupports.cmake)