Fix compilation of natpmp-helper on non-windows

Fixes a small oversight in 5dbfb1b3.

changes/bug4691

+  o Major bugfixes:
+    - Fix compilation of the libnatpmp helper on non-Windows. Bugfix on
+      0.2.3.9-alpha; fixes bug 4691. Reported by Anthony G. Basile.
+

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

@@ -16,6 +16,9 @@
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
+#ifndef MS_WINDOWS
+#include <arpa/inet.h>
+#endif
 
 // debugging stuff
 #include <assert.h>