Opps, include SerialFlash in VolumeRamp example (for Arduino < 1.6.7)

examples/HardwareTesting/SGTL5000/VolumeRamp/VolumeRamp.ino

@@ -12,6 +12,7 @@
 #include <Wire.h>
 #include <SPI.h>
 #include <SD.h>
+#include <SerialFlash.h>
 
 // GUItool: begin automatically generated code
 AudioSynthWaveformSine   sine1;          //xy=203,233