bridge: Allow to easy change network namespace

Added new '-netns' option to simplify executing following cmd:

    ip netns exec NETNS bridge OPTIONS COMMAND OBJECT

    to

    bridge -n[etns] NETNS OPTIONS COMMAND OBJECT

e.g.:

    bridge -net vnet0 fdb

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
diff --git a/bridge/Makefile b/bridge/Makefile
index 1fb8320..9800753 100644
--- a/bridge/Makefile
+++ b/bridge/Makefile
@@ -2,6 +2,10 @@
 
 include ../Config
 
+ifeq ($(IP_CONFIG_SETNS),y)
+	CFLAGS += -DHAVE_SETNS
+endif
+
 all: bridge
 
 bridge: $(BROBJ) $(LIBNETLINK)