Warning redirecting incorrect #include <sys/poll.h> to <poll.h>

[mojca]

When compiling luasocket on Alpine Linux I get the following warning

/usr/include/sys/poll.h:1:2: warning: redirecting incorrect #include <sys/poll.h> to <poll.h>

and in fact after some web searching it seems that poll.h should probably be used instead of sys/poll.h.