lib: libc: arcmwdt: add a declaration for the strnlen function

ARC MWDT C library does not have an implementation of the strnlen
function, so we have a wrapper over standard-compliant strnlen_s.
However, there is no declaration of the wrapper, so add one.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>