Stack: Use non-amalgamated headers in all the examples

For consistency we recommend using non-amalgamation install, thus the
examples should also use the non-amalgamated headers.

examples/tutorial_server_variable.c

@@ -14,9 +14,13 @@
  * read only. See below for making the variable writable.
  */
 
-#include "open62541.h"
+#include <ua_server.h>
+#include <ua_config_default.h>
+#include <ua_log_stdout.h>
+
 #include <signal.h>
 
+
 static void
 addVariable(UA_Server *server) {
     /* Define the attribute of the myInteger variable node */