drivers: wifi: eswifi: Specify POSIX-2008 to use `strnlen()`

`_POSIX_C_SOURCE` must be specified to use the `strnlen()` function
in string.h. Make sure to specify this explicitly.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

drivers/wifi/eswifi/eswifi_core.c

@@ -10,6 +10,8 @@
 #define DT_DRV_COMPAT inventek_eswifi
 #endif
 
+#define _POSIX_C_SOURCE 200809L
+
 #include "eswifi_log.h"
 LOG_MODULE_REGISTER(LOG_MODULE_NAME);