libnetbsd: Avoid bringing in all of sys/param.h in sys/types.h

It's okay to hard-code NBBY, as the value is somewhat unlikely to
change.

The pollution from sys/param.h makes it harder to import test code from
NetBSD since it can introduce conflicting definitions, e.g.,
MIN()/MAX().

Reviewed by:	ngie
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D57754

lib/libnetbsd/sys/types.h

@@ -31,6 +31,6 @@
 
 #include_next <sys/types.h>
 
-#include <sys/param.h>	/* For NBBY */
+#define	NBBY	8
 
 #endif