process.h: add a struct declaration for buf_t.

This lets us avoid a dependency on include order.

src/lib/process/process.h

@@ -127,18 +127,19 @@ void process_notify_event_exit(process_t *process,
                                process_exit_code_t);
 
 #ifdef PROCESS_PRIVATE
-MOCK_DECL(STATIC int, process_read_stdout, (process_t *, buf_t *));
-MOCK_DECL(STATIC int, process_read_stderr, (process_t *, buf_t *));
-MOCK_DECL(STATIC void, process_write_stdin, (process_t *, buf_t *));
+struct buf_t;
+MOCK_DECL(STATIC int, process_read_stdout, (process_t *, struct buf_t *));
+MOCK_DECL(STATIC int, process_read_stderr, (process_t *, struct buf_t *));
+MOCK_DECL(STATIC void, process_write_stdin, (process_t *, struct buf_t *));
 
 STATIC void process_read_data(process_t *process,
-                              buf_t *buffer,
+                              struct buf_t *buffer,
                               process_read_callback_t callback);
 STATIC void process_read_buffer(process_t *process,
-                                buf_t *buffer,
+                                struct buf_t *buffer,
                                 process_read_callback_t callback);
 STATIC void process_read_lines(process_t *process,
-                               buf_t *buffer,
+                               struct buf_t *buffer,
                                process_read_callback_t callback);
 #endif /* defined(PROCESS_PRIVATE) */