[build] split out DefaultFileSource and dependents to a separate target

We don't want to link it into the node bindings, so keep it in a separate target

src/mbgl/util/http_timeout.cpp

@@ -1,6 +1,8 @@
 #include <mbgl/util/http_timeout.hpp>
 #include <mbgl/util/constants.hpp>
 
+#include <cassert>
+
 namespace mbgl {
 namespace http {