include: export lapd.h since openBSC needs it

More specifically, the unfinished ericsson rbs2000 BTS.
diff --git a/src/input/lapd.c b/src/input/lapd.c
index 6c070e3..2e0dd08 100644
--- a/src/input/lapd.c
+++ b/src/input/lapd.c
@@ -36,7 +36,7 @@
 #include <assert.h>
 #include <errno.h>
 
-#include "lapd.h"
+#include <osmocom/abis/lapd.h>
 
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/core/logging.h>