emit_spirv_instructions: Add missing header guard

src/shader_recompiler/backend/spirv/emit_spirv_instructions.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <sirit/sirit.h>
 
 #include "common/common_types.h"