Add missing ipconfigUSE_DNS_CACHE check

Description

In FreeRTOS_DNS_Globals.h the freertos_addrinfo struct adds in a member ucName when using ipCONFIG_USE_DNS_CACHE:

/** @brief freertos_addrinfo is the equivalent of 'struct addrinfo'. */
    struct freertos_addrinfo
    {
        BaseType_t ai_flags;                /**< The field is included for completeness, but it is not used. */
        BaseType_t ai_family;               /**< The type of IP-address, either FREERTOS_AF_INET4 or FREERTOS_AF_INET6. */
        BaseType_t ai_socktype;             /**< n.a. */
        BaseType_t ai_protocol;             /**< n.a. */
        socklen_t ai_addrlen;               /**< The length of the address, either ipSIZE_OF_IPv4_ADDRESS or ipSIZE_OF_IPv6_ADDRESS. */
        struct freertos_sockaddr * ai_addr; /**< The socket address. */
        char * ai_canonname;                /**< The name of the host. */
        struct freertos_addrinfo * ai_next; /**< A pointer to the next find result, or NULL. */
        struct
        {
            /* In order to avoid allocations, reserve space here for *ai_addr and *ai_canonname. */
            struct freertos_sockaddr sockaddr;

            #if ( ipconfigUSE_DNS_CACHE != 0 )
                char ucName[ ipconfigDNS_CACHE_NAME_LENGTH ];
            #endif
        }
        xPrivateStorage; /**< In order to avoid extra calls to malloc, the necessary space is reserved 'statically'. */
    };

The ucName variable is currently used without performing this check in FreeRTOS_DNS.c:pxNew_AddrInfo():

pxAddrInfo->ai_canonname = pxAddrInfo->xPrivateStorage.ucName;
( void ) strncpy( pxAddrInfo->xPrivateStorage.ucName, pcName, sizeof( pxAddrInfo->xPrivateStorage.ucName ) - 1U );
pxAddrInfo->xPrivateStorage.ucName[ sizeof( pxAddrInfo->xPrivateStorage.ucName ) - 1U ] = '\0';

This PR wraps the usage of the ucName variable in a #if ( ipconfigUSE_DNS_CACHE != 0 ) block.

Test Steps

Perform a build with the following options set:

#define ipconfigUSE_DNS                   ipconfigENABLE
#define ipconfigUSE_DNS_CACHE             ipconfigDISABLE

Receive a build error as in FreeRTOS_DNS.c there is no check for if ipCONFIG_USE_DNS_CACHE is enabled.

Perform the build with this change added without error.

Checklist:

[x] I have tested my changes. No regression in existing tests.
[ ] I have modified and/or added unit-tests to cover the code changes in this Pull Request.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

FreeRTOS / FreeRTOS-Plus-TCP