src: move includes inside include guard

Having the includes in src/node_buffer.h outside of the include guard
is not really harmful but it's inconsistent with other header files.

src/node_buffer.h

@@ -19,13 +19,13 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+#ifndef SRC_NODE_BUFFER_H_
+#define SRC_NODE_BUFFER_H_
+
 #include "node.h"
 #include "smalloc.h"
 #include "v8.h"
 
-#ifndef SRC_NODE_BUFFER_H_
-#define SRC_NODE_BUFFER_H_
-
 namespace node {
 namespace Buffer {