Use KJ_SYSCALL_FD.

Turns out this is the only file in this repo that uses KJ_SYSCALL at all!

src/workerd/server/workerd.c++

@@ -200,9 +200,7 @@ class FileWatcher {
   kj::HashMap<int, kj::HashSet<kj::String>> filesWatched;
 
   static kj::AutoCloseFd makeInotify() {
-    int fd;
-    KJ_SYSCALL(fd = inotify_init1(IN_NONBLOCK | IN_CLOEXEC));
-    return kj::AutoCloseFd(fd);
+    return KJ_SYSCALL_FD(inotify_init1(IN_NONBLOCK | IN_CLOEXEC));
   }
 };
 
@@ -233,17 +231,13 @@ class FileWatcher {
       KJ_IF_SOME(fd, f.getFd()) {
         // We need to duplicate the FD because the original will probably be closed later and
         // closing the FD unregisters it from kqueue.
-        int duped;
-        KJ_SYSCALL(duped = dup(fd));
-        watchFd(kj::AutoCloseFd(duped));
+        watchFd(KJ_SYSCALL_FD(dup(fd)));
         return;
       }
     }
 
     // No existing file, open from disk.
-    int fd;
-    KJ_SYSCALL(fd = open(path.toNativeString(true).cStr(), O_RDONLY));
-    watchFd(kj::AutoCloseFd(fd));
+    watchFd(KJ_SYSCALL_FD(open(path.toNativeString(true).cStr(), O_RDONLY)));
   }
 
   kj::Promise<void> onChange() {
@@ -275,9 +269,7 @@ class FileWatcher {
   kj::Vector<kj::AutoCloseFd> filesWatched;
 
   static kj::AutoCloseFd makeKqueue() {
-    int fd_;
-    KJ_SYSCALL(fd_ = kqueue());
-    auto fd = kj::AutoCloseFd(fd_);
+    auto fd = KJ_SYSCALL_FD(kqueue());
     KJ_SYSCALL(fcntl(fd, F_SETFD, FD_CLOEXEC));
     return kj::mv(fd);
   }