Fix build on FreeBSD

[yzgyyang]

The include directive of netinet/in.h must be before resolv.h on FreeBSD, otherwise, the following errors will occur:

In file included from dns.c:265:
/usr/include/resolv.h:159:14: error: array has incomplete element type 'struct sockaddr_in'
                nsaddr_list[MAXNS];     /*%< address of name server */
                           ^
/usr/include/resolv.h:158:9: note: forward declaration of 'struct sockaddr_in'
        struct sockaddr_in
               ^
/usr/include/resolv.h:173:18: error: field has incomplete type 'struct in_addr'
                struct in_addr  addr;
                                ^
/usr/include/resolv.h:173:10: note: forward declaration of 'struct in_addr'
                struct in_addr  addr;
                       ^
/usr/include/resolv.h:198:21: error: field has incomplete type 'struct sockaddr_in'
        struct sockaddr_in      sin;
                                ^
/usr/include/resolv.h:158:9: note: forward declaration of 'struct sockaddr_in'
        struct sockaddr_in
               ^
3 errors generated.

The reason is that the resolv.h on FreeBSD is not yet self-contained, see this bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=182466

This fix was committed with the FreeBSD port net/R-cran-pingr in https://cgit.freebsd.org/ports/commit/?id=fce80f530301d0079d84bc4b818ba957ad29617b.