core/dns: Add system include required on FreeBSD

While testing the latest RC on FreeBSD I noticed this new file fails to build. On FreeBSD inlcuding resolv.h requires sockaddr_in to be defined, and it's defined in netinet/in.h. So I added this include.

ASTERISK-28853 #close

Change-Id: I6997daf3956e6eb70ab6cb358628d162fad80079

main/dns_txt.c

@@ -27,6 +27,7 @@
 
 #include "asterisk.h"
 
+#include <netinet/in.h>
 #include <resolv.h>
 
 #include "asterisk/dns_core.h"