Added SD MMC support.

[Zhentao-Lin]

1，In AudioFileSourceSD.h, the header file for calling the SD card is #include <SD.h>. However, if I use SD_MMC to call the SD card, I won't be able to use it, I have to change it to #include <SD_MMC.h>.

2，In AudioFileSourceSD.cpp, the open function is as follows: *bool AudioFileSourceSD::open(const char filename) { f = SD.open(filename, FILE_READ); return f; } However, if I use SD_MMC to call the SD card, I will not be able to use it, I have to change to: *bool AudioFileSourceSD::open(const char filename) { f = SD_MMC.open(filename, FILE_READ); return f; } Nothing else in the library needs to change. Will you consider adding the SD_MMC feature to support?