net: ocpp: Correct way of getting strdup prototype

Zephyr subsystems' headers should not duplicate C library prototypes
(in this case strdup(), which is either ISO C23 or a POSIX extension to
the C library <string.h>).

Instead they should request those prototypes from the C library.
By now Zephyr only requires ISO C17, but many C libraries will have
strdup() and expose it also when the POSIX extensions prototypes are
requested, so let's request these prototype from the C library by
setting the feature test macro _POSIX_C_SOURCE to version 2008.09
which includes it.

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