test_rtttl_pcm: include Arduino.h first to fix the native link

Without it this file's setup()/loop() definitions get plain C++
linkage while portduino's main.cpp expects Arduino.h's declarations,
and the coverage build fails with 'undefined reference to setup/loop'
(other suites include it transitively via meshUtils.h/TestUtil.h).
Fixes the failing test-native CI job on this PR; suite runs 30/30.

test/test_rtttl_pcm/test_main.cpp

@@ -1,3 +1,8 @@
+// Arduino.h must come first: it declares setup()/loop() with the linkage portduino's
+// main.cpp expects. Without it this file's definitions get C++ mangling and the
+// coverage build fails to link (RtttlPcm.h itself is deliberately Arduino-free).
+#include <Arduino.h>
+
 #include "audio/RtttlPcm.h"
 #include <cstring>
 #include <string>