build: glibc on Linux defines explicit_bzero in <string.h>.

Don't install strings.h on such systems. The local prototype doesn't
match the native Linux one when built as C++ breaking the build of
config(8) as a bootstrap tool on some Linux systems. If the missing
prototype becomes a problem in the future,
tools/build/cross-build/include/linux/strings.h can be patched to add
it.

Reviewed by: imp, emaste
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D39289