Added missing declaration to DSLBlock.h

DSLBlock references SymbolTable without declaring it, getting away with
it only because it was normally imported by other headers.

Change-Id: Iead5c65656604d1c5b65ac3231ee3172350d28ed
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/417676
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/include/sksl/DSLBlock.h b/include/sksl/DSLBlock.h
index c20c864..c2ad7de 100644
--- a/include/sksl/DSLBlock.h
+++ b/include/sksl/DSLBlock.h
@@ -16,6 +16,7 @@
 
 namespace SkSL {
 
+class SymbolTable;
 class Statement;
 
 namespace dsl {