General: Resolve warnings related to missing declarations

src/video_core/textures/decoders.cpp

@@ -11,6 +11,7 @@
 #include "video_core/textures/texture.h"
 
 namespace Tegra::Texture {
+namespace {
 
 /**
  * This table represents the internal swizzle of a gob,
@@ -174,6 +175,8 @@ void SwizzledData(u8* const swizzled_data, u8* const unswizzled_data, const bool
     }
 }
 
+} // Anonymous namespace
+
 void CopySwizzledData(u32 width, u32 height, u32 depth, u32 bytes_per_pixel,
                       u32 out_bytes_per_pixel, u8* const swizzled_data, u8* const unswizzled_data,
                       bool unswizzle, u32 block_height, u32 block_depth, u32 width_spacing) {