gitmodules: update mce-test repo URL

According to mce-test maintainer Chen Gong, the project repo URL has
been moved to git.kernel.org, so update the URL.

Signed-off-by: Caspar Zhang <caspar@casparzhang.com>
diff --git a/.gitmodules b/.gitmodules
index e9eefdb..1c9e9c3 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "testcases/kernel/mce-test"]
 	path = testcases/kernel/mce-test
-	url = git://github.com/andikleen/mce-test.git
+	url = git://git.kernel.org/pub/scm/linux/kernel/git/gong.chen/mce-test.git