fix various compiler warnings (on FreeBSD-11.0)

FreeBSD 11.0 uses clang version 3.8.0 which spits various warnings
during libosmocore compilation.  Let's clean this up a bit.

Change-Id: Ic14572e6970bd0b8916604fabf807f1608fa07e5
diff --git a/src/gsm/abis_nm.c b/src/gsm/abis_nm.c
index 10aae9a..155084a 100644
--- a/src/gsm/abis_nm.c
+++ b/src/gsm/abis_nm.c
@@ -571,7 +571,7 @@
 		return NULL;
 	}
 	if (len)
-		msgb_tl16v_put(nmsg, NM_ATT_ADD_TEXT, len, add_text);
+		msgb_tl16v_put(nmsg, NM_ATT_ADD_TEXT, len, (const uint8_t *) add_text);
 
 	return nmsg;
 }