include: export lapd.h since openBSC needs it

More specifically, the unfinished ericsson rbs2000 BTS.
diff --git a/include/osmocom/abis/Makefile.am b/include/osmocom/abis/Makefile.am
index 85c77b7..ea1eded 100644
--- a/include/osmocom/abis/Makefile.am
+++ b/include/osmocom/abis/Makefile.am
@@ -1,5 +1,6 @@
 osmoabis_HEADERS = abis.h		\
 		   e1_input.h		\
+		   lapd.h		\
 		   logging.h		\
 		   subchan_demux.h	\
 		   ipa_proxy.h		\