Drop a bunch of SYN_DROPPED related structs on the stack
Remove them from the libevdev-internal device struct and let's keep them on the stack instead, it makes things a lot easier.
This used to all be pre-allocated because most of libevdev is sigsafe but so is our stack, so we might as well not allocate this on the heap to begin with.
cc @bentiss if you can find the time to throw some eyeballs at this, thanks