modules/zcbor: Fix implicit function declaration warning

Both zcbor_encode.c & zcbor_decode.c use strnlen()
In general these 2 functions are only exposed by the C library
if _POSIX_C_SOURCE is set 200809L.
But neither of these files (or their build scripts), are setting
this macro, causing build warnings
 Implicit declaration of function ‘strnlen’
which turn into failures in CI with some libCs.
Let's set this macro to avoid this issue.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>