Fix rustdoc unresolved import error if libsystemd feature is disabled

Author: SpriteOvO

.github/workflows/ci.yml

@@ -126,8 +126,10 @@ jobs:
       run: |
         cd spdlog
         # The reason for allowing lint `unexpected_cfgs` not in `build.rs` or `Cargo.toml` but in CI is that we need to keep MSRV
+        cargo +nightly rustdoc --verbose -- -D warnings -A unexpected_cfgs -A rustdoc::broken-intra-doc-links
         cargo +nightly rustdoc --all-features --verbose -- -D warnings -A unexpected_cfgs
         cd ../spdlog-macros
+        cargo +nightly rustdoc --verbose -- -D warnings -A unexpected_cfgs -A rustdoc::broken-intra-doc-links
         cargo +nightly rustdoc --all-features --verbose -- -D warnings -A unexpected_cfgs
 
   check-msrv:

spdlog/src/sink/journald_sink.rs

@@ -47,7 +47,7 @@ impl Default for SyslogLevels {
 }
 
 fn journal_send(args: impl Iterator<Item = impl AsRef<str>>) -> StdResult<(), io::Error> {
-    #[cfg(target_os = "linux")] // https://github.com/rust-lang/rust/issues/97976
+    #[cfg(not(doc))] // https://github.com/rust-lang/rust/issues/97976
     use libsystemd_sys::{const_iovec, journal as ffi};
 
     let iovecs: Vec<_> = args.map(|a| unsafe { const_iovec::from_str(a) }).collect();