vircgroupv1: fix build on non-linux OSes

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: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>