lte_connectivity: fix LTE_CONNECTIVITY build with CONFIG_POSIX_API=y

inet_pton() function is only accessible when:
 * CONFIG_POSIX_API=y or
 * CONFIG_NET_SOCKETS_POSIX_NAMES=y
are enabled.

Depending on the selected option (both are mutually exclusive), its
definition is located in different header file.
`lte_ip_addr_helper.c` (part of LTE_CONNECTIVITY) is including just one
of those, assuming CONFIG_NET_SOCKETS_POSIX_NAMES=y.

Add compatibility with CONFIG_POSIX_API=y by including `arpa/inet.h`
whenever POSIX API is enabled.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

lib/nrf_modem_lib/lte_connectivity/lte_ip_addr_helper.c

@@ -12,6 +12,10 @@
 #include <nrf_modem_at.h>
 #include <string.h>
 
+#ifdef CONFIG_POSIX_API
+#include <arpa/inet.h>
+#endif
+
 #include <zephyr/logging/log.h>
 
 LOG_MODULE_REGISTER(ip_addr_helper, CONFIG_LTE_CONNECTIVITY_LOG_LEVEL);