export the 'trx_is_usable() function and use it from paging

this ensures we don't send paging requests to currently inactive transceivers.
diff --git a/openbsc/src/chan_alloc.c b/openbsc/src/chan_alloc.c
index abf4de8..8141e47 100644
--- a/openbsc/src/chan_alloc.c
+++ b/openbsc/src/chan_alloc.c
@@ -47,7 +47,7 @@
 	return 1;
 }
 
-static int trx_is_usable(struct gsm_bts_trx *trx)
+int trx_is_usable(struct gsm_bts_trx *trx)
 {
 	/* FIXME: How does this behave for BS-11 ? */
 	if (is_ipaccess_bts(trx->bts)) {