Fix a format-security warning when building with clang. (ros2#1064)

In particular, you should never have a "bare" string
in a printf-like call; that could potentially access
uninitialized memory.  Instead, make sure to format
the string with %s.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rcl/test/rcl/test_logging_rosout.cpp

@@ -194,7 +194,7 @@ check_if_rosout_subscription_gets_a_message(
   size_t iteration = 0;
   const char * message = "SOMETHING";
   do {
-    RCUTILS_LOG_INFO_NAMED(logger_name, message);
+    RCUTILS_LOG_INFO_NAMED(logger_name, "%s", message);
     ++iteration;
     ret = rcl_wait_set_clear(&wait_set);
     ASSERT_EQ(RCL_RET_OK, ret) << rcl_get_error_string().str;