Shell: Moves pipelined processes to one process group

Author: https://github.com/alimpfard
Commit: ab3e787334
Pull-request: https://github.com/SerenityOS/serenity/pull/3085
Reviewed-by: https://github.com/BenWiederhake

Shell/Forward.h

@@ -32,5 +32,6 @@ namespace AST {
 class Node;
 class Value;
 class SyntaxError;
+class Pipeline;
 
 }