Fix buildroot branch

We've been on 2018.11 for some time now.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 621ac42..e128ce3 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2018.08-op-build
+	branch = 2018.11-op-build
 	url = https://github.com/open-power/buildroot