LibGfx: Add a method to ImageDecoderPlugin for reading ICC data

This probably won't be the final API for getting color spaces
from images, since some formats just store an "is sRGB?" flag
instead of a full profile. Instead, once everything works,
we probably want to give every Bitmap a pointer to some
color space abstraction.

But we can always change this later, once things are further along
and better understood.

Userland/Libraries/LibGfx/ICOLoader.cpp

@@ -278,4 +278,9 @@ ErrorOr<ImageFrameDescriptor> ICOImageDecoderPlugin::frame(size_t index)
     return ImageFrameDescriptor { m_context->images[m_context->largest_index].bitmap, 0 };
 }
 
+ErrorOr<Optional<ReadonlyBytes>> ICOImageDecoderPlugin::icc_data()
+{
+    return OptionalNone {};
+}
+
 }