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/mncc.c b/openbsc/src/mncc.c
index 8cd62f6..f62541c 100644
--- a/openbsc/src/mncc.c
+++ b/openbsc/src/mncc.c
@@ -30,7 +30,7 @@
 #include <openbsc/talloc.h>
 #include <openbsc/gsm_data.h>
 
-static void *tall_call_ctx;
+void *tall_call_ctx;
 
 static struct mncc_names {
 	char *name;
@@ -389,8 +389,3 @@
 
 	return rc;
 }
-
-static __attribute__((constructor)) void on_dso_load_trau_mncc(void)
-{
-	tall_call_ctx = talloc_named_const(tall_bsc_ctx, 1, "gsm_call");
-}