Common: Move header guards over to pragma once

Also replaced C headers with the C++ equivalent ones

src/common/linear_disk_cache.h

@@ -2,9 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-
-#ifndef _LINEAR_DISKCACHE
-#define _LINEAR_DISKCACHE
+#pragma once
 
 #include "common/common.h"
 #include <fstream>
@@ -187,5 +185,3 @@ private:
     std::fstream m_file;
     u32 m_num_entries;
 };
-
-#endif  // _LINEAR_DISKCACHE