video_core/textures/convert: Replace include with a forward declaration

Avoids dragging in a direct dependency in a header.

src/video_core/textures/convert.cpp

@@ -10,6 +10,7 @@
 #include "common/assert.h"
 #include "common/common_types.h"
 #include "common/logging/log.h"
+#include "video_core/surface.h"
 #include "video_core/textures/astc.h"
 #include "video_core/textures/convert.h"
 

src/video_core/textures/convert.h

@@ -5,7 +5,10 @@
 #pragma once
 
 #include "common/common_types.h"
-#include "video_core/surface.h"
+
+namespace VideoCore::Surface {
+enum class PixelFormat;
+}
 
 namespace Tegra::Texture {