script/check-config.sh: add OVERLAY_FS check

While this is used by the majority of upper container runtimes, it was
not needed for runc itself. Since commit 515f09f7 runc uses overlay,
too, so let's add a check for this.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

script/check-config.sh

@@ -241,6 +241,9 @@ flags=(
 
 	# required for bind-mounting /dev/mqueue into containers
 	POSIX_MQUEUE
+
+	# Most containers use overlayfs, and now runc itself uses it.
+	OVERLAY_FS
 )
 check_flags "${flags[@]}"