internal: Move <stdio.h> include to internal.h

It doesn't really make sense for us to have stdlib.h and string.h but
not stdio.h in the internal.h header.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>

tests/storagebackendsheepdogtest.c

@@ -23,7 +23,6 @@
 
 #include <config.h>
 
-#include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>