gitmodules: Fix to include lz4.

.gitmodules

@@ -31,3 +31,6 @@
 [submodule "cpr"]
     path = externals/cpr
     url = https://github.com/whoshuu/cpr.git
+[submodule "lz4"]
+    path = externals/lz4
+    url = http://github.com/lz4/lz4.git