node-api: move NAPI_EXPERIMENTAL definition to gyp file

Author: gabrielschulhof

test/js-native-api/test_string/binding.gyp

@@ -5,7 +5,10 @@
       "sources": [
         "test_string.c",
         "test_null.c",
-      ]
-    }
-  ]
+      ],
+      "defines": [
+        "NAPI_EXPERIMENTAL",
+      ],
+    },
+  ],
 }

test/js-native-api/test_string/test_string.c

@@ -1,8 +1,7 @@
+#include <js_native_api.h>
 #include <limits.h>  // INT_MAX
 #include <stdlib.h>
 #include <string.h>
-#define NAPI_EXPERIMENTAL
-#include <js_native_api.h>
 #include "../common.h"
 #include "../entry_point.h"
 #include "test_null.h"