Made SkSL DSL into public API

In addition to the unsurprising changes to eliminate references to
src/, we also had to tighten up some C++17-isms as they are not
permitted in public headers.

Change-Id: Ie5005a33d7a135e69fb66beca5e7a5f960dbd453
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378496
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/src/sksl/SkSLProgramSettings.h b/src/sksl/SkSLProgramSettings.h
index 0253a9c..8b5dab3 100644
--- a/src/sksl/SkSLProgramSettings.h
+++ b/src/sksl/SkSLProgramSettings.h
@@ -8,7 +8,7 @@
 #ifndef SKSL_PROGRAMSETTINGS
 #define SKSL_PROGRAMSETTINGS
 
-#include "src/sksl/SkSLDefines.h"
+#include "include/private/SkSLDefines.h"
 
 namespace SkSL {