nat: Move creation of the structs to a separate file
This way one can create the bsc_nat structure in unit tests..
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 9aa9948..2f2b8c9 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -135,6 +135,8 @@
/* create and init the structures */
struct bsc_config *bsc_config_alloc(struct bsc_nat *nat, const char *token, unsigned int lac);
struct bsc_config *bsc_config_num(struct bsc_nat *nat, int num);
+struct bsc_nat *bsc_nat_alloc(void);
+struct bsc_connection *bsc_connection_alloc(struct bsc_nat *nat);
/**