BUILD: namespaces: fix a potential build warning in namespaces.c

I just met this warning today making me realize that haproxy's
headers were included prior to the system ones, so all #ifndefs
are taken first then the system redefines them. Simply move
haproxy includes after the system's. This should be backported
to 1.6 as well.

In file included from /usr/include/bits/fcntl.h:61:0,
                 from /usr/include/fcntl.h:35,
                 from src/namespace.c:13:
/usr/include/bits/fcntl-linux.h:203:0: warning: "F_SETPIPE_SZ" redefined [enabled by default]
In file included from include/common/config.h:26:0,
                 from include/proto/log.h:29,
                 from src/namespace.c:7:
include/common/compat.h:81:0: note: this is the location of the previous definition
diff --git a/src/namespace.c b/src/namespace.c
index f1e81df..108c994 100644
--- a/src/namespace.c
+++ b/src/namespace.c
@@ -1,12 +1,5 @@
 #define _GNU_SOURCE
 
-#include <common/namespace.h>
-#include <common/compiler.h>
-#include <common/hash.h>
-#include <common/errors.h>
-#include <proto/log.h>
-#include <types/global.h>
-
 #include <sched.h>
 #include <stdio.h>
 #include <sys/stat.h>
@@ -16,6 +9,14 @@
 #include <sys/socket.h>
 
 #include <string.h>
+
+#include <common/namespace.h>
+#include <common/compiler.h>
+#include <common/hash.h>
+#include <common/errors.h>
+#include <proto/log.h>
+#include <types/global.h>
+
 #ifdef CONFIG_HAP_NS
 
 /* Opens the namespace <ns_name> and returns the FD or -1 in case of error