Fix building on OS X (missing NULL in queue.h)
Fix building on OS X (missing NULL in queue.h)

Headers seem to be a bit different in Apple land and you need to include
stddef.h explicitly to the NULL declaration.

This also makes the code a bit more correct, as stddef.h is the header
in the C standard that defines NULL
(https://en.cppreference.com/w/cpp/header/cstddef).

Genymobile/scrcpy,Genymobile/scrcpy,Genymobile/scrcpy