move talloc context creation out of on_dso / constructors

the various constructors get called in a non-obvious, linker determined
order, which makes certain objects disappear from the talloc report.

This change moves the talloc context creation into a new talloc_ctx.c file
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index a75ef7a..abf073c 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -2199,7 +2199,7 @@
 
 /* BS11 SWL */
 
-static void *tall_fle_ctx;
+void *tall_fle_ctx;
 
 struct abis_nm_bs11_sw {
 	struct gsm_bts *bts;
@@ -2564,10 +2564,3 @@
 {
 	return __simple_cmd(bts, NM_MT_IPACC_RESTART);
 }
-
-
-static __attribute__((constructor)) void on_dso_load_abis_nm(void)
-{
-	tall_fle_ctx = talloc_named_const(tall_bsc_ctx, 1, 
-					  "bs11_file_list_entry");
-}