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>
1 file changed