Include poll.h instead of sys/poll.h

`poll.h` is the POSIX standard include location, and MUSL
libc issues a warning (which infnoise's `-Werror` in CFLAGS
makes into an error).

For more context:
- [Linux manpage for `poll(2)`][1]
- [Open Group UNIX Specification][2]
- [Relevant discussion over at the `emscripten` project][3]

[1]: https://man7.org/linux/man-pages/man2/poll.2.html
[2]: https://pubs.opengroup.org/onlinepubs/7908799/xsh/poll.h.html
[3]: https://github.com/emscripten-core/emscripten/issues/5447#issuecomment-321513332

software/writeentropy.c

@@ -4,7 +4,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/ioctl.h>
 #include <fcntl.h>
 #include <linux/random.h>