Replace ifdef guards with #pragma once

tl;dr this has potentially significant compilation speed benefits.

src/NanoManager.hpp

@@ -1,5 +1,4 @@
-#ifndef _NM_HPP
-#define _NM_HPP
+#pragma once
 
 #include "CNShardServer.hpp"
 
@@ -17,5 +16,3 @@ namespace NanoManager {
     void setNanoSkill(CNSocket* sock, int16_t nanoId, int16_t skillId);
     void resetNanoSkill(CNSocket* sock, int16_t nanoId);
 }
-
-#endif