MINOR: task: stop including stream.h from task.c

This one comes with a very deep dependency hell, only to know that
process_stream() is a function. Dropping it reduces the preprocessed
output from 1.5MB to 640kB.

src/task.c

@@ -20,10 +20,10 @@
 #include <haproxy/fd.h>
 #include <haproxy/list.h>
 #include <haproxy/pool.h>
-#include <haproxy/stream.h>
 #include <haproxy/task.h>
 #include <haproxy/tools.h>
 
+extern struct task *process_stream(struct task *t, void *context, unsigned int state);
 
 DECLARE_POOL(pool_head_task,    "task",    sizeof(struct task));
 DECLARE_POOL(pool_head_tasklet, "tasklet", sizeof(struct tasklet));