Make header files self-sufficient

Change-Id: Ice7d761b1023da77e50e5d6aa597964f7d9aa1d8
Reviewed-on: https://skia-review.googlesource.com/8302
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/src/sksl/ast/SkSLASTParameter.h b/src/sksl/ast/SkSLASTParameter.h
index 7b653ad..6bb13ac 100644
--- a/src/sksl/ast/SkSLASTParameter.h
+++ b/src/sksl/ast/SkSLASTParameter.h
@@ -8,6 +8,7 @@
 #ifndef SKSL_ASTPARAMETER
 #define SKSL_ASTPARAMETER
 
+#include "SkSLASTPositionNode.h"
 #include "SkSLASTType.h"
 #include "../ir/SkSLModifiers.h"