nat: Allocate a named context to make dumping allocations possible

This is fixing the SIGUSR1 to really report the allocated
memory on stderr.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 5284427..0462f9d 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -818,6 +818,8 @@
 
 int main(int argc, char** argv)
 {
+	tall_bsc_ctx = talloc_named_const(NULL, 1, "nat");
+
 	log_init(&log_info);
 	stderr_target = log_target_create_stderr();
 	log_add_target(stderr_target);