netutils/ntpclient: fix a warning if CONFIG_NETUTILS_DHCPC not defined

[masc2008]

Summary

This change fixes a warning in netutils/ntpclient/ntpclient.c when
CONFIG_NETUTILS_DHCPC is not enabled.

ntpc_set_servers_from_dhcp() is only used from the DHCP notification
path, but it and its local helper ntpc_dup_server_list() were compiled
unconditionally. In non-DHCP builds this can leave unused static helpers
and trigger compiler warnings.

This patch wraps the DHCP-only declaration and helper definitions with
#ifdef CONFIG_NETUTILS_DHCPC and keeps the shared startup logic
unchanged. ntpc_select_server_list() remains unconditional because it
is still used by the normal ntpc_start() path.

Impact

This change is limited to netutils/ntpclient.

It does not change runtime behavior when CONFIG_NETUTILS_DHCPC is
enabled. For builds without CONFIG_NETUTILS_DHCPC, it removes
unnecessary DHCP-only code from compilation and avoids warning noise.

There is no API change and no documentation update is needed for this
internal warning fix.

Testing

Build-tested change only.

Host:

• Ubuntu Linux
• Compiler: GCC

Target / config:

• a configuration with CONFIG_NETUTILS_DHCPC=y
• a configuration with CONFIG_NETUTILS_DHCPC not set

Verification:

• confirmed the DHCP-enabled build still compiles with the DHCP callback path
• confirmed the non-DHCP build no longer compiles the DHCP-only helpers

Runtime testing on hardware was not performed for this warning fix.

[masc2008]

sorry to introduced this warning, not sure if this needs to be fixed.