Cgroups are linux specific and we need to make sure that the code is
compiled only on linux.  On different OSes it fails the compilation:

../../src/util/vircgroupv1.c:65:19: error: variable has incomplete type 'struct mntent'
    struct mntent entry;
                  ^
../../src/util/vircgroupv1.c:65:12: note: forward declaration of 'struct mntent'
    struct mntent entry;
           ^
../../src/util/vircgroupv1.c:74:12: error: implicit declaration of function 'getmntent_r' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    while (getmntent_r(mounts, &entry, buf, sizeof(buf)) != NULL) {
           ^
../../src/util/vircgroupv1.c:814:39: error: use of undeclared identifier 'MS_NOSUID'
    if (mount("tmpfs", root, "tmpfs", MS_NOSUID|MS_NODEV|MS_NOEXEC, opts) < 0) {
                                      ^
../../src/util/vircgroupv1.c:814:49: error: use of undeclared identifier 'MS_NODEV'
    if (mount("tmpfs", root, "tmpfs", MS_NOSUID|MS_NODEV|MS_NOEXEC, opts) < 0) {
                                                ^
../../src/util/vircgroupv1.c:814:58: error: use of undeclared identifier 'MS_NOEXEC'
    if (mount("tmpfs", root, "tmpfs", MS_NOSUID|MS_NODEV|MS_NOEXEC, opts) < 0) {
                                                         ^
../../src/util/vircgroupv1.c:841:65: error: use of undeclared identifier 'MS_BIND'
            if (mount(src, group->legacy[i].mountPoint, "none", MS_BIND,
                                                                ^
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>