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.
(cherry picked from commit 97207ac8)
(cherry picked from commit 2fb61237)