Update Arduino_GigaDisplay_GFX.h

[Dowster64]

Added rtos.h to the include files section to as the type rtos cannot be found and the examples fail to compile without this change.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}