move msgb talloc context into talloc_ctx.c

This keeps the ordering right, independent of link ordering, it
will ensure that the talloc dump will also catch msgb related
memory leaks.
diff --git a/openbsc/src/msgb.c b/openbsc/src/msgb.c
index 48a5a7b..9f2fcfa 100644
--- a/openbsc/src/msgb.c
+++ b/openbsc/src/msgb.c
@@ -28,7 +28,7 @@
 #include <openbsc/talloc.h>
 #include <openbsc/debug.h>
 
-static void *tall_msgb_ctx;
+void *tall_msgb_ctx;
 
 struct msgb *msgb_alloc(u_int16_t size, const char *name)
 {
@@ -96,8 +96,3 @@
 	msg->l3h = NULL;
 	msg->smsh = NULL;
 }
-
-static __attribute__((constructor)) void on_dso_load_trau_msgb(void)
-{
-	tall_msgb_ctx = talloc_named_const(tall_bsc_ctx, 1, "msgb");
-}