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/sksl/codegen/SkSLVMCodeGenerator.h b/src/sksl/codegen/SkSLVMCodeGenerator.h
index 2ef8f9b..fa0ce16 100644
--- a/src/sksl/codegen/SkSLVMCodeGenerator.h
+++ b/src/sksl/codegen/SkSLVMCodeGenerator.h
@@ -8,8 +8,8 @@
 #ifndef SKSL_VMGENERATOR
 #define SKSL_VMGENERATOR
 
+#include "include/core/SkSpan.h"
 #include "include/private/SkSLString.h"
-#include "src/core/SkSpan.h"
 #include "src/core/SkVM.h"
 #include "src/sksl/ir/SkSLType.h"