Clear namespace pollution in include/malloc_np.h

Do not include stdbool.h, it makes the header incompatible with some
third-party code that typedefs bool manually.
Remove inclusion of strings.h, which typically conflicts with the use
of symbol 'index'.
Separate inclusion of sys/cdefs.h is not needed because sys/types.h
already handles that.

Exp-run by: antoine (PR 245366)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24297