cli: Move some declarations into `cli.h`

This will help to keep new CLIs clean from internal headers.

Change-Id: I3e5515ae5645fcdce56c13df1ff23de829bbbdb9
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/72989
diff --git a/cli_output.c b/cli_output.c
index 5f02fc3..88ee93a 100644
--- a/cli_output.c
+++ b/cli_output.c
@@ -19,7 +19,9 @@
 #include <stdarg.h>
 #include <string.h>
 #include <errno.h>
+
 #include "flash.h"
+#include "cli.h"
 
 enum flashprog_log_level verbose_screen = FLASHPROG_MSG_INFO;
 enum flashprog_log_level verbose_logfile = FLASHPROG_MSG_DEBUG2;