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/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 2994d72..85ad0ec 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -53,7 +53,7 @@
 
 #define UM_SAPI_SMS 3	/* See GSM 04.05/04.06 */
 
-static void *tall_gsms_ctx;
+void *tall_gsms_ctx;
 static u_int32_t new_callref = 0x40000001;
 
 struct value_string {
@@ -1076,7 +1076,5 @@
 
 static __attribute__((constructor)) void on_dso_load_sms(void)
 {
-	tall_gsms_ctx = talloc_named_const(tall_bsc_ctx, 1, "sms");
-
 	register_signal_handler(SS_SUBSCR, subscr_sig_cb, NULL);
 }