include: export lapd.h since openBSC needs it

More specifically, the unfinished ericsson rbs2000 BTS.
diff --git a/include/Makefile.am b/include/Makefile.am
index c94d3ee..ea38eec 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -1,3 +1,3 @@
-noinst_HEADERS=mISDNif.h internal.h lapd.h
+noinst_HEADERS=mISDNif.h internal.h
 
 SUBDIRS=osmocom