Expose `get_builtin_module` function

This is a requirement for electron/electron#8815, which exposes some builtin
modules to sandboxed renderers.

(cherry picked from commit 4c058a6cb5331eef1c7f51c042dd074a2d4f9234)

src/node.h

@@ -403,6 +403,7 @@ struct node_module {
   struct node_module* nm_link;
 };
 
+NODE_EXTERN node_module* get_builtin_module(const char *name);
 extern "C" NODE_EXTERN void node_module_register(void* mod);
 
 #ifdef _WIN32