try to fix compiling with gcc & clang

src/core/ipv6/ip6_addr.c

@@ -233,7 +233,7 @@ ip6addr_ntoa_r(const ip6_addr_t *addr, char *buf, int buflen)
 #define IP4MAPPED_HEADER "::FFFF:"
     char *buf_ip4 = buf + sizeof(IP4MAPPED_HEADER) - 1;
     int buflen_ip4 = buflen - sizeof(IP4MAPPED_HEADER) + 1;
-    if (buflen < sizeof(IP4MAPPED_HEADER)) {
+    if (buflen < (int)sizeof(IP4MAPPED_HEADER)) {
       return NULL;
     }
     memcpy(buf, IP4MAPPED_HEADER, sizeof(IP4MAPPED_HEADER));

test/unit/udp/test_udp.c

@@ -149,7 +149,7 @@ START_TEST(test_udp_new_remove)
 }
 END_TEST
 
-void test_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p,
+static void test_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p,
     const ip_addr_t *addr, u16_t port)
 {
   struct test_udp_rxdata *ctr = (struct test_udp_rxdata *)arg;