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/ir/SkSLPrefixExpression.h b/src/sksl/ir/SkSLPrefixExpression.h
index b7db99a..dafe1e9 100644
--- a/src/sksl/ir/SkSLPrefixExpression.h
+++ b/src/sksl/ir/SkSLPrefixExpression.h
@@ -9,6 +9,7 @@
 #define SKSL_PREFIXEXPRESSION
 
 #include "SkSLExpression.h"
+#include "SkSLToken.h"
 
 namespace SkSL {