Add the SdFat library.  It can be downloaded in the 1.8.5 IDE using the Library Manager.  It has the missing header you are looking for.

Thank you for posting the new library!!!!  It works great.  I am curious as to why TinyCircuits hasn't updated the documentation and provided a repo to pull this library from?

For anyone who tries the library example program: Be sure to add the SdFat library.  It can be downloaded in the 1.8.5 IDE using the Library Manager.  It is needed for the example code to function.

I am seeing this error as well with the 1.8.4 Arduino IDE on my mac.  Does anyone else have this problem or have a possible solution?  Below is the error that is showing up in my IDE.
Arduino: 1.8.4 (Mac OS X), Board: "TinyScreen+, Default"

In file included from XXXXX/Documents/Arduino/libraries/WaveHC/WaveHC.cpp:15:0:
XXXXX/Documents/Arduino/libraries/WaveHC/mcpDac.h:26:20: fatal error: avr/io.h: No such file or directory
#include <avr/io.h>

                    ^
compilation terminated.
exit status 1
Error compiling for board TinyScreen+.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.