Use correct stat struct for ftw()

Use correct stat struct for ftw()

check_dir_mtime() is called by ftw() and is given
the real stat struct, not its glib version (which may
or may not be the same as "struct stat").

This is irrelevant for MSVC (it has no ftw()) and
works correctly for MinGW-w64 (which declares stat
structures correctly). If mingw.org complains, add
a special ifdef for it later.

Origin: upstream, 3.24.39, commit:7e17693cced0db855bfe0997f26633ca8bdd48cc

Gbp-Pq: Name Use-correct-stat-struct-for-ftw.patch