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

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