Isolation: wired up cgroup to build system.

This commit enables the building of the cgroup code. This is only built
when the cgroupv2 filesystem is found.

If cgroupv2 support is found then

  cgroupv2: .................. YES

will be printed by ./configure

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/sources

@@ -304,6 +304,11 @@ if [ "$NXT_HAVE_CLONE" = "YES" ]; then
 fi
 
 
+if [ "$NXT_HAVE_CGROUP" = "YES" ]; then
+    NXT_LIB_SRCS="$NXT_LIB_SRCS src/nxt_cgroup.c"
+fi
+
+
 if [ "$NXT_TEST_BUILD" = "YES" ]; then
     NXT_LIB_SRCS="$NXT_LIB_SRCS $NXT_TEST_BUILD_SRCS"
 fi