Move SkSpan to include/, for use in public API

Change-Id: I674f038600afd6d49316c1ece515941ee5579068
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/406939
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrDirectContextPriv.h b/src/gpu/GrDirectContextPriv.h
index a6a8c02..caa4d7a 100644
--- a/src/gpu/GrDirectContextPriv.h
+++ b/src/gpu/GrDirectContextPriv.h
@@ -8,9 +8,9 @@
 #ifndef GrDirectContextPriv_DEFINED
 #define GrDirectContextPriv_DEFINED
 
+#include "include/core/SkSpan.h"
 #include "include/core/SkSurface.h"
 #include "include/gpu/GrDirectContext.h"
-#include "src/core/SkSpan.h"
 
 class GrAtlasManager;
 class GrBackendFormat;