[misc] Fix make distcheck by adding compat_af_isdn.h
diff --git a/openbsc/include/Makefile.am b/openbsc/include/Makefile.am
index 36d57a3..a95129f 100644
--- a/openbsc/include/Makefile.am
+++ b/openbsc/include/Makefile.am
@@ -1,3 +1,3 @@
 SUBDIRS = openbsc vty
 
-noinst_HEADERS = mISDNif.h
+noinst_HEADERS = mISDNif.h compat_af_isdn.h