(via Mbed OS) nanostack: icmpv6: fix build warning

Fix the following build warning seen when building with GCC

Compile [ 54.2%]: icmpv6.c
[Warning] icmpv6.c@1091,16: this statement may fall through [-Wimplicit-fallthrough=]

Author: Naveen Kaje

source/Common_Protocols/icmpv6.c

@@ -1106,15 +1106,15 @@ buffer_t *icmpv6_up(buffer_t *buf)
 
         case ICMPV6_TYPE_INFO_ECHO_REPLY:
             ipv6_neighbour_reachability_confirmation(buf->src_sa.address, buf->interface->id);
-        /* fall through */
+            /* fall through */
 
         case ICMPV6_TYPE_ERROR_DESTINATION_UNREACH:
 #ifdef HAVE_RPL_ROOT
             if (buf->options.type == ICMPV6_TYPE_ERROR_DESTINATION_UNREACH && buf->options.code == ICMPV6_CODE_DST_UNREACH_SRC_RTE_HDR_ERR) {
                 buf = rpl_control_source_route_error_handler(buf, cur);
             }
 #endif
-        /* fall through */
+            /* fall through */
 
         default:
             if (buf) {