shader: add missing include guard in half_floating_point_helper.h

src/shader_recompiler/frontend/maxwell/translate/impl/half_floating_point_helper.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include "common/common_types.h"
 #include "shader_recompiler/exception.h"
 #include "shader_recompiler/frontend/maxwell/translate/impl/common_encoding.h"