treewide: fix sys/poll.h -> poll.h

poll() and pollfd are defined in poll.h per POSIX¹, and musl complains²
about it.

[1]: https://man.archlinux.org/man/poll.h.0p
[2]: https://github.com/kraj/musl/blob/8cb84492b0245d70b2cd0edd523e2b55c7ad67a9/include/sys/poll.h#L1


Change-Id: I4fff447394903cbf55e330f07ed82b906a6a6964

lix/libutil/cgroup.cc

@@ -5,7 +5,7 @@
 #include <deque>
 #include <fcntl.h>
 #include <filesystem>
-#include <sys/poll.h>
+#include <poll.h>
 #if __linux__
 
 #include "regex.hh"

lix/libutil/file-descriptor.cc

@@ -8,7 +8,7 @@
 
 #include <cerrno>
 #include <fcntl.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/socket.h>
 #include <sys/syscall.h>
 #include <unistd.h>

tests/unit/libstore/filetransfer.cc

@@ -14,7 +14,7 @@
 #include <netinet/in.h>
 #include <string>
 #include <string_view>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/socket.h>
 #include <thread>
 #include <unistd.h>