video_core: Add missing header guards

Ensures errors related to multiple inclusions never happen.

src/video_core/renderer_opengl/gl_shader_decompiler.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <array>
 #include <functional>
 #include <optional>

src/video_core/swrasterizer/proctex.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 "common/vector_math.h"
 #include "video_core/pica_state.h"