shims: flesh out generic_sys_queue.h further
shims: flesh out generic_sys_queue.h further

The 1121 merge introduced additional usage of sys/queue.h.  Flesh out
the shims header further to repair the build on Windows.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch