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/subchan_demux.c b/openbsc/src/subchan_demux.c
index 368b985..63be533 100644
--- a/openbsc/src/subchan_demux.c
+++ b/openbsc/src/subchan_demux.c
@@ -31,7 +31,7 @@
 #include <openbsc/talloc.h>
 #include <openbsc/gsm_data.h>
 
-static void *tall_tqe_ctx;
+void *tall_tqe_ctx;
 
 static inline void append_bit(struct demux_subch *sch, u_int8_t bit)
 {
@@ -320,9 +320,3 @@
 
 	return 0;
 }
-
-static __attribute__((constructor)) void on_dso_load_ss_demux(void)
-{
-	tall_tqe_ctx = talloc_named_const(tall_bsc_ctx, 1,
-					  "subch_txq_entry");
-}