Restore sys/random.h inclusion for getentropy()
Code movement for the refactoring for ticket 24658 didn't copy the
inclusion of sys/random.h, which is needed to get a prototype for
getentropy() on macOS 10.12 Sierra. It also didn't copy the inclusion
of sys/syscall.h, which might prevent the getrandom() syscall from
being properly detected. Move these inclusions. Bug not in any
released Tor.