xxHash: add .gitignore for build artifacts

Signed-off-by: Sage Weil <sage@redhat.com>

diff --git a/src/xxHash b/src/xxHash
index 44a6297b298e59ab7452defe859f21ed8371aa1c..1f40c6511fa8dd9d2e337ca8c9bc18b3e87663c9 160000 (submodule)
--- a/src/xxHash
+++ b/src/xxHash
@@ -1 +1 @@
-Subproject commit 44a6297b298e59ab7452defe859f21ed8371aa1c
+Subproject commit 1f40c6511fa8dd9d2e337ca8c9bc18b3e87663c9