BUILD: makefile: enable linux namespaces by default on linux

Oldest kernel found on a supported Linux distro (2.6.32 + backports on
RHEL6) supports network namespaces, so we have no reason not to enable
them by default on the linux-glibc target.
diff --git a/Makefile b/Makefile
index d56ca43..a4ec9bf 100644
--- a/Makefile
+++ b/Makefile
@@ -323,7 +323,7 @@
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
     USE_CPU_AFFINITY USE_THREAD USE_EPOLL USE_FUTEX USE_LINUX_TPROXY          \
-    USE_ACCEPT4 USE_LINUX_SPLICE USE_PRCTL USE_THREAD_DUMP)
+    USE_ACCEPT4 USE_LINUX_SPLICE USE_PRCTL USE_THREAD_DUMP USE_NS)
 endif
 
 # Solaris 8 and above