Fix BSD `glibtop_get_netload: Function not implemented` error

This fixes #22 (closed) by adding glibtop_init() when plugin inits.

It is Ok when only use one command from libgtop, whether glibtop_get_netlist() or glibtop_get_netload() works fine when they are the only command used by one program. but combinations could trigger Function not implemented error.

This bug seems not triggered on Linux, but all BSDs might be affected (OpenBSD/NetBSD/FreeBSD/Darwin).

Reproduce:

// cc -DCASE_1 reproduce.c -o test_netlist
// cc -DCASE_2 reproduce.c -o test_netload
// cc -DUSE_BOTH reproduce.c -o test_combine
// cc -DINIT -DUSE_BOTH reproduce.c -o test_combine_with_init

// test_combine is expected to trigger error:
//   [ERROR] glibtop_get_netload: Function not implemented

#include <stdio.h>
#include <glibtop/netlist.h>
#include <glibtop/netload.h>

int main()
{
    glibtop_netload netload;
    glibtop_netlist netlist;

#ifdef INIT
    glibtop_init();
#endif

#if defined(CASE_1)
    char **interfaces = glibtop_get_netlist (&netlist);
    for (int i = 0; i < netlist.number; ++i) {
        printf("interfaces[%d]: %s\n", i, interfaces[i]);
    }
#elif defined(CASE_2)
    glibtop_get_netload (&netload, "en0");
    printf("netload.bytes_total: %llu\n", netload.bytes_total);
#elif defined(USE_BOTH)
    char **interfaces = glibtop_get_netlist (&netlist);
    for (int i = 0; i < netlist.number; ++i) {
        glibtop_get_netload (&netload, interfaces[i]);
        printf("%s: netload.bytes_total: %llu\n", interfaces[i], netload.bytes_total);
    }
#endif

    glibtop_close();

    return 0;
}