umdns: add missing syscall to seccomp filter

The 'madvise', syscall is missing.
Found with 'utrace /usr/sbin/umdns' on an R7800 and RT3200.

Signed-off-by: Michael Peleshenko <mpeleshenko@gmail.com>

package/network/services/umdns/files/umdns.json

@@ -21,6 +21,7 @@
 				"fstat",
 				"getsockname",
 				"ioctl",
+				"madvise",
 				"mmap",
 				"mmap2",
 				"munmap",