Make lwip_stack.c include its header

Fix some type mismatches that this revealed.

Author: kjbracey

features/FEATURE_LWIP/lwip-interface/lwip_stack.c

@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <stdbool.h>
 #include <string.h>
+#include "lwip_stack.h"
 
 #include "eth_arch.h"
 #include "lwip/opt.h"
@@ -391,7 +392,7 @@ char *mbed_lwip_get_ip_address(char *buf, nsapi_size_t buflen)
 #endif
 }
 
-const char *mbed_lwip_get_netmask(char *buf, nsapi_size_t buflen)
+char *mbed_lwip_get_netmask(char *buf, nsapi_size_t buflen)
 {
 #if LWIP_IPV4
     const ip4_addr_t *addr = netif_ip4_netmask(&lwip_netif);

features/FEATURE_LWIP/lwip-interface/lwip_stack.h

@@ -31,9 +31,9 @@ nsapi_error_t mbed_lwip_bringup(bool dhcp, bool ppp, const char *ip, const char
 nsapi_error_t mbed_lwip_bringdown(bool ppp);
 
 const char *mbed_lwip_get_mac_address(void);
-char *mbed_lwip_get_ip_address(char *buf, int buflen);
-char *mbed_lwip_get_netmask(char *buf, int buflen);
-char *mbed_lwip_get_gateway(char *buf, int buflen);
+char *mbed_lwip_get_ip_address(char *buf, nsapi_size_t buflen);
+char *mbed_lwip_get_netmask(char *buf, nsapi_size_t buflen);
+char *mbed_lwip_get_gateway(char *buf, nsapi_size_t buflen);
 
 extern nsapi_stack_t lwip_stack;