core: increase stack size of init-child entrypoint

With the commit "init: session-label rewriting", the stack usage
increased due to the handling of session-label strings as local
variables. The stack overrun occurred in the vmm scenario on
base-hw.

repos/base/src/core/main.cc

@@ -121,7 +121,7 @@ class Core_child : public Child_policy
 		 * Entry point used for serving the parent interface
 		 */
 		Rpc_entrypoint _entrypoint;
-		enum { STACK_SIZE = 2 * 1024 * sizeof(Genode::addr_t)};
+		enum { STACK_SIZE = 4 * 1024 * sizeof(Genode::addr_t)};
 
 		Registry<Service> &_services;