Update stoip6 prototype to tests (ARMmbed#1851)

Author: Arto Kinnunen

test/nanostack/unittest/stub/ip6tos_stub.c

@@ -28,11 +28,16 @@ uint_fast8_t ip6_prefix_tos(const void *prefix, uint_fast8_t prefix_len, char *p
     return 0;
 }
 
-void stoip6(const char *ip6addr, size_t len, void *dest)
+bool stoip6(const char *ip6addr, size_t len, void *dest)
 {
 }
 
 unsigned char sipv6_prefixlength(const char *ip6addr)
 {
     return 0;
 }
+
+int stoip6_prefix(const char *ip6addr, void *dest, int_fast16_t *prefix_len_out)
+{
+    return 0;
+}