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?