Add `WDF_STRUCTURE_SIZE` macro equivalent

[wmmc88]

WDF_STRUCTURE_SIZE is a macro available in the WDK, but isn't available in wdk-sys since bindgen can't generate macros. WDF_STRUCTURE_SIZE is useful since many of the _INIT macros are also not available right now. A 1:1 implementation of WDF_STRUCTURE_SIZE should be made available in wdk-sys, although an equivalent shouldn't be needed in wdk given that the safe abstractions shouldn't need to do any of the c-style manual initializations.

[0xflux]

I'd like to have a look at this @wmmc88. Would this live in wdk-sys/ntddk.rs?

[wmmc88]

I'd like to have a look at this @wmmc88. Would this live in wdk-sys/ntddk.rs?

It would live in wdf.rs, however i would hold off on doing anything here yet since we're investigating whether we can look into using bindgen's new clang_macro_fallback feature

[0xflux]

Ok no probs! Looking for some places to contribute so I'll keep my eyes open