libutil: fix linux build on fresh glibc and gcc

Without the change the build fails for me as:

    ../unix/file-descriptor.cc:404:70: error: 'RESOLVE_BENEATH' was not declared in this scope
      404 |         dirFd, path.rel_c_str(), flags, static_cast<uint64_t>(mode), RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS);
          |                                                                      ^~~~~~~~~~~~~~~

This happens for 2 reasons:
1. `__NR_openat2` constant was not pulled in from the according headers
   and as a result `<linux/openat2.h>` was not included.
2. `define HAVE_OPENAT2 0` build is broken: refers to missing
   `RESOLVE_BENEATH` normally pulled in from `<linux/openat2.h>`

This changes fixes both.

src/libutil/unix/file-descriptor.cc

@@ -9,9 +9,12 @@
 #include <unistd.h>
 #include <poll.h>
 
+#if defined(__linux__)
+#  include <sys/syscall.h> /* pull __NR_* definitions */
+#endif
+
 #if defined(__linux__) && defined(__NR_openat2)
 #  define HAVE_OPENAT2 1
-#  include <sys/syscall.h>
 #  include <linux/openat2.h>
 #else
 #  define HAVE_OPENAT2 0
@@ -399,7 +402,7 @@ Descriptor unix::openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPa
 {
     assert(!path.rel().starts_with('/')); /* Just in case the invariant is somehow broken. */
     assert(!path.isRoot());
-#ifdef __linux__
+#if HAVE_OPENAT2
     auto maybeFd = linux::openat2(
         dirFd, path.rel_c_str(), flags, static_cast<uint64_t>(mode), RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS);
     if (maybeFd) {