REORG: include: move common/namespace.h to haproxy/namespace{,-t}.h

The type was moved out as it's used by standard.h for netns_entry.
Instead of just being a forward declaration when not used, it's an
empty struct, which makes gdb happier (the resulting stripped executable
is the same).
diff --git a/src/namespace.c b/src/namespace.c
index 6165994..4f82c62 100644
--- a/src/namespace.c
+++ b/src/namespace.c
@@ -11,7 +11,7 @@
 #include <string.h>
 
 #include <haproxy/api.h>
-#include <common/namespace.h>
+#include <haproxy/namespace.h>
 #include <haproxy/hash.h>
 #include <haproxy/errors.h>
 #include <proto/log.h>