core: Fix time namespace in RestrictNamespaces=

RestrictNamespaces= would accept "time" but would not actually apply
seccomp filters e.g. systemd-run -p RestrictNamespaces=time unshare -T true
should fail but it succeeded.

This commit actually enables time namespace seccomp filtering.

src/shared/nsflags.h

@@ -14,7 +14,8 @@
                           CLONE_NEWNS|                                  \
                           CLONE_NEWPID|                                 \
                           CLONE_NEWUSER|                                \
-                          CLONE_NEWUTS))
+                          CLONE_NEWUTS|                                 \
+                          CLONE_NEWTIME))
 
 #define NAMESPACE_FLAGS_INITIAL  ULONG_MAX