kernel-iph: Add a stub for a Windows IP Helper based networking backend

src/libcharon/Makefile.am

@@ -496,6 +496,13 @@ if MONOLITHIC
 endif
 endif
 
+if USE_KERNEL_IPH
+  SUBDIRS += plugins/kernel_iph
+if MONOLITHIC
+  libcharon_la_LIBADD += plugins/kernel_iph/libstrongswan-kernel-iph.la
+endif
+endif
+
 if USE_WHITELIST
   SUBDIRS += plugins/whitelist
 if MONOLITHIC