Why header "linux/if_ppp.h" returns errors?

Question

I would like to implement the solution of user @bunto1 from this post (What is the easiest way to get PPP0 interface Tx/Rx bytes in a custom C/C++ program?) in my code:

It consists in reading RX and TX from the "ppp0" interface.

    #include <linux/if_ppp.h>
    #include <net/if.h>
    #include <sys/ioctl.h>
    #include <sys/socket.h>
    #include <unistd.h>

Code:

    auto sockfd = ::socket(PF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        std::cout << "couldn't open socket : " << errno << std::endl;
        return;
    }

    ifreq req {};
    ppp_stats stats {};

    req.ifr_data = reinterpret_cast<caddr_t>(&stats);
    ::strncpy(&req.ifr_name[0], "ppp0", sizeof(req.ifr_name));

    auto ret = ::ioctl(sockfd, SIOCGPPPSTATS, &req);
    if (ret < 0) {
        std::cout << "couldn't get PPP statistics : " << errno << std::endl;
    } else {
        std::cout << "received bytes : " << stats.p.ppp_ibytes << std::endl;
        std::cout << "sent bytes : " << stats.p.ppp_obytes << std::endl;
    }

    auto ret = ::close(sockfd);
    if (ret != 0) {
        std::cout << "couldn't close socket : " << errno << std::endl;
    }

Unfortunately, when I wrote this code for myself, the compiler reports an error:

/usr/include/linux/if_ppp.h:92:7: error: use of enum ‘NPmode’ without previous declaration
  enum NPmode mode;
       ^
/usr/include/linux/if_ppp.h:103:16: error: field ‘b’ has incomplete type ‘ifreq’
  struct ifreq  b;
                ^
/usr/include/linux/if_ppp.h:104:19: error: field ‘stats’ has incomplete type ‘ppp_stats’
  struct ppp_stats stats;   /* statistic information */
                   ^
/usr/include/linux/if_ppp.h:108:21: error: field ‘b’ has incomplete type ‘ifreq’
  struct ifreq       b;
                     ^
usr/include/linux/if_ppp.h:109:24: error: field ‘stats’ has incomplete type ‘ppp_comp_stats’
  struct ppp_comp_stats stats;

I noticed that this is due to the inclusion of a header file:

#include <linux/if_ppp.h>

What could this be caused by?