netmap.h/freebsd: Include <atomic> instead of <stdatomic.h> when compiling with C++
netmap.h/freebsd: Include <atomic> instead of <stdatomic.h> when compiling with C++

Compiling the following C++ program on FreeBSD (11.4 / 12.1) currently fails.
This affects the netmap plugin for Zeek [1].

    $ cat test_netmap.cc
    #include <iostream>

    extern "C" {
    #define NETMAP_WITH_LIBS
    #include <net/netmap_user.h>
    }

    int main(int argc, char *argv[])
    {
            std::cout << "netmap" << std::endl;
    }

    $ clang++  ./test_netmap.cc
    In file included from ./test_netmap.cc:5:
    In file included from /usr/include/net/netmap_user.h:100:
    In file included from /usr/include/net/netmap.h:814:
    /usr/include/stdatomic.h:187:17: error: unknown type name '_Bool'
    typedef _Atomic(_Bool)                  atomic_bool;
                    ^
    /usr/include/stdatomic.h:187:26: error: C++ requires a type specifier for all declarations
    typedef _Atomic(_Bool)                  atomic_bool;

Similar issues [2,3] (though GCC) suggest to use <atomic> instead
of <stdatomic.h> for C++.

[1] https://github.com/zeek/bro-netmap/issues/11
[2] https://bugs.python.org/issue23644
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60932#c4

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap