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)
(cherry picked from commit e88aec2c)
(cherry picked from commit 9ef259dd)