Add `WDF_STRUCTURE_SIZE` macro equivalent

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.