build: fix isisd topology out of tree build

isisd got the include directory wrong on building with a separate build
directory.

  * configure.ac: adjust ISIS_TOPOLOGY_INCLUDES

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/configure.ac b/configure.ac
index 0d6e6ef..7764df7 100755
--- a/configure.ac
+++ b/configure.ac
@@ -330,7 +330,7 @@
 
 if test "${enable_isisd}" = "yes" && test "${enable_isis_topology}" = yes; then
   AC_DEFINE(TOPOLOGY_GENERATE,,Enable IS-IS topology generator code)
-  ISIS_TOPOLOGY_INCLUDES="-I./topology"
+  ISIS_TOPOLOGY_INCLUDES="-I\$(srcdir)/topology"
   ISIS_TOPOLOGY_DIR="topology"
   ISIS_TOPOLOGY_LIB="./topology/libtopology.a"
 fi