src: add e1_input_vty.c to libosmo-abis

This file provides the VTY interface for E1 lines, I have moved
it to libosmo-abis since it belongs here.
diff --git a/include/osmocom/abis/e1_input.h b/include/osmocom/abis/e1_input.h
index 4339e05..51c9273 100644
--- a/include/osmocom/abis/e1_input.h
+++ b/include/osmocom/abis/e1_input.h
@@ -216,6 +216,8 @@
 void e1inp_sign_link_destroy(struct e1inp_sign_link *link);
 int e1inp_line_update(struct e1inp_line *line);
 
+int e1inp_vty_init(void);
+
 struct gsm_network;
 int ipaccess_setup(struct gsm_network *gsmnet);
 int hsl_setup(struct gsm_network *gsmnet);