flow: Add missing include, stdio.h

Reported-by: lemmi <lemmi@nerd2nerd.org>
Link: https://github.com/void-linux/void-packages/actions/runs/7097192513/job/19316903568
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow.c

@@ -6,6 +6,7 @@
  */
 
 #include <stdint.h>
+#include <stdio.h>
 #include <unistd.h>
 #include <string.h>