base: move 'prepare_init_main_thread' to Genode::

Issue #4784

repos/base-linux/src/lib/base/thread_env.cc

@@ -18,8 +18,12 @@
 
 #include <linux_syscalls.h>
 
+/* base-internal includes */
+#include <base/internal/globals.h>
+
 using namespace Genode;
 
+
 extern addr_t * __initial_sp;
 
 /*
@@ -73,7 +77,7 @@ void lx_exception_signal_handlers()
  ** Startup library support **
  *****************************/
 
-void prepare_init_main_thread()
+void Genode::prepare_init_main_thread()
 {
 	/*
 	 * Initialize the 'lx_environ' pointer