nat: Fix compiler warning about unknown prototype

bsc_nat_ctrl.c: In function ‘set_net_cfg_cmd’:
bsc_nat_ctrl.c:360:3: warning: implicit declaration of function ‘bsc_replace_string’ [-Wimplicit-function-declaration]
   bsc_replace_string(bsc_cfg, &bsc_cfg->acc_lst_name, cmd->value);
   ^

openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c

@@ -27,6 +27,7 @@
 
 #include <openbsc/ctrl.h>
 #include <openbsc/bsc_nat.h>
+#include <openbsc/vty.h>
 
 #include <unistd.h>
 #include <string.h>