Add SPDX.

Author: kattni

Adafruit_STEMMA_Audio_Amp/Arduino/PWMAudio_Mono_RP2040/.qt_py_rp2040.test.only renamed to Adafruit_STEMMA_Audio_Amp/Arduino/PWMAudio_Mono_RP2040/.feather_rp2040.test.only

@@ -1,3 +1,8 @@
+// SPDX-FileCopyrightText: 2023 Kattni Rembor for Adafruit Industries
+// SPDX-FileCopyrightText: Earle F. Philhower, III
+//
+// SPDX-License-Identifier: MIT
+
 /*
   This example plays a tune through a mono amplifier using a simple sine wave.