core: set private section name for automount units

Because this was left unset, the unit_write_setting() function was
refusing to write out the automount-specific TimeoutIdleSec= and
DirectoryMode= settings when creating transient automount units.
Set it to the proper value in line with other unit types.

src/core/automount.c

@@ -1085,6 +1085,7 @@ const UnitVTable automount_vtable = {
                 "Unit\0"
                 "Automount\0"
                 "Install\0",
+        .private_section = "Automount",
 
         .can_transient = true,
         .can_fail = true,