Include compat.h from natpmp-helper to fix build

This means tor_socket_t is declared.

changes/bug4955

@@ -0,0 +1,5 @@
+  o Major bugfixes:
+    - Fix the build when built with nat-pmp support. Fixes bug 4955;
+      bugfix on 0.2.3.11-alpha. Reported by Anthony G. Basile
+      <blueness@gentoo.org>, thanks!
+

src/tools/tor-fw-helper/tor-fw-helper-natpmp.c

@@ -23,6 +23,8 @@
 // debugging stuff
 #include <assert.h>
 
+#include "compat.h"
+
 #include "tor-fw-helper.h"
 #include "tor-fw-helper-natpmp.h"