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/paging.c b/openbsc/src/paging.c
index dd22150..802024a 100644
--- a/openbsc/src/paging.c
+++ b/openbsc/src/paging.c
@@ -49,7 +49,7 @@
 #define PAGING_TIMEOUT 1, 75000
 #define MAX_PAGING_REQUEST 750
 
-static void *tall_paging_ctx;
+void *tall_paging_ctx;
 
 static unsigned int calculate_group(struct gsm_bts *bts, struct gsm_subscriber *subscr)
 {
@@ -309,8 +309,3 @@
 {
 	bts->paging.available_slots = free_slots;
 }
-
-static __attribute__((constructor)) void on_dso_load_paging(void)
-{
-	tall_paging_ctx = talloc_named_const(NULL, 1, "paging_request");
-}