shared: include <stdlib.h> in "nm-macros-internal.h"

shared: include <stdlib.h> in "nm-macros-internal.h"

"nm-macros-internal.h" uses free() for the "nm_auto_free"
macro. Thus, as long as that code is there, we anyway must
include <stdlib.h> along the line.

Do it in "nm-macros-internal.h" to make the header self-contained.