update SPL submodule to 0.7.4-1

diff --git a/spl-debian b/spl-debian
index 785f8eda81c22b7fdf35a078bfe2e7533d4727b7..9936e726fc593f37f54079905886a1c903ca7220 160000 (submodule)
--- a/spl-debian
+++ b/spl-debian
@@ -1 +1 @@
-Subproject commit 785f8eda81c22b7fdf35a078bfe2e7533d4727b7
+Subproject commit 9936e726fc593f37f54079905886a1c903ca7220