kernel/memory: Remove #pragma once within cpp file

This isn't necessary in a cpp file and will cause warnings on clang.

src/core/hle/kernel/memory/system_control.cpp

@@ -2,8 +2,6 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#pragma once
-
 #include <random>
 
 #include "core/hle/kernel/memory/system_control.h"