Any reason not to put this code in main.c? Same for the tests in the next commit.
The Win32 LE GATT API has nothing in common with the classic one, including separate headers, device interfaces and IOCTLs. For that reason, I thought it'd be more appropriate to split the code. gatt.c will likely be a larger file than main.c (for both the dll and tests) in the future, once we start achieving API parity.