auth: Add missing include/auth.h

Doh !

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/openbsc/include/openbsc/Makefile.am b/openbsc/include/openbsc/Makefile.am
index 0805f05..2c5db5b 100644
--- a/openbsc/include/openbsc/Makefile.am
+++ b/openbsc/include/openbsc/Makefile.am
@@ -9,7 +9,7 @@
 		 vty.h socket.h \
 		crc24.h gprs_bssgp.h gprs_llc.h gprs_ns.h gprs_gmm.h \
 		gb_proxy.h gprs_sgsn.h gsm_04_08_gprs.h sgsn.h \
-		gprs_ns_frgre.h
+		gprs_ns_frgre.h auth.h
 
 openbsc_HEADERS = gsm_04_08.h meas_rep.h bsc_api.h
 openbscdir = $(includedir)/openbsc
diff --git a/openbsc/include/openbsc/auth.h b/openbsc/include/openbsc/auth.h
new file mode 100644
index 0000000..0e5cad6
--- /dev/null
+++ b/openbsc/include/openbsc/auth.h
@@ -0,0 +1,10 @@
+#ifndef _AUTH_H
+#define _AUTH_H
+
+struct gsm_auth_tuple;
+struct gsm_subscriber;
+
+int auth_get_tuple_for_subscr(struct gsm_auth_tuple *atuple,
+                              struct gsm_subscriber *subscr, int key_seq);
+
+#endif /* _AUTH_H */