histedit.h: Add missing #include <sys/cdefs.h>

This fixes building openssh on the bootstrap image.

The real problem is that it picks up this file instead of the histedit.h
from libedit, though. But since this include was missing anyway, it
makes sense to fix this file, too.

headers/compatibility/bsd/histedit.h

@@ -40,6 +40,7 @@
 #ifndef _HISTEDIT_H_
 #define	_HISTEDIT_H_
 
+#include <sys/cdefs.h>
 #include <sys/types.h>
 #include <stdio.h>