osbuild2: files input is a valid copy stage input

Mark the `FilesInput` as valid input for the copy stage input so it
can be used in the constructor helper function(s).

Co-Developed-by: Achilleas Koutsou <achilleas@koutsou.net>
Co-Developed-by: Antonio Murdaca <runcom@linux.com>

internal/osbuild2/files_input.go

@@ -24,6 +24,9 @@ func NewFilesInputs(references FilesInputReferences) *FilesInputs {
 // inputs accepted by the XZ stage
 func (FilesInputs) isXzStageInputs() {}
 
+// inputs accepted by the Copy stage
+func (FilesInputs) isCopyStageInputs() {}
+
 // SPECIFIC INPUT STRUCTURE
 
 type FilesInput struct {