login/logind: Include sys/stat.h for struct stat usage

We need to include `<sys/stat.h>` for usage of the `struct stat` in
the Manager struct, much as we already include `<stdbool.h>` for C99
booleans.

This helps alleviate another minor build failure on non-glibc systems.

src/login/logind.h

@@ -2,6 +2,7 @@
 #pragma once
 
 #include <stdbool.h>
+#include <sys/stat.h>
 
 #include "sd-bus.h"
 #include "sd-device.h"