LibSQL: Add missing forward declarations

Author: https://github.com/trflynn89
Commit: 35f0450dd8
Pull-request: https://github.com/SerenityOS/serenity/pull/6602
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibSQL/Forward.h

@@ -24,9 +24,11 @@ class Delete;
 class DropTable;
 class ErrorExpression;
 class ErrorStatement;
+class ExistsExpression;
 class Expression;
 class GroupByClause;
 class InChainedExpression;
+class InSelectionExpression;
 class InTableExpression;
 class InvertibleNestedDoubleExpression;
 class InvertibleNestedExpression;