use installed steam_api.h

Index: src/common.hpp
--- src/common.hpp.orig
+++ src/common.hpp
@@ -1,7 +1,7 @@
 #ifndef LUASTEAM_COMMON
 #define LUASTEAM_COMMON
 
-#include "../sdk/public/steam/steam_api.h"
+#include <steam_api.h>
 extern "C" {
 #include <lauxlib.h>
 #include <lua.h>
