debian: bump compat level to 11

Increase the compat level to 11 since we're now getting warnings about
level 9 being obsolescent.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/debian/compat b/debian/compat
index ec63514..b4de394 100644
--- a/debian/compat
+++ b/debian/compat
@@ -1 +1 @@
-9
+11