posix: Declare fstat, stat, lstat and fstatat as public
posix: Declare fstat, stat, lstat and fstatat as public

Fixes https://gitlab.gnome.org/GNOME/vala/issues/1086

GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala