debian/watch: Add options for signature validation

Fixes lintian warning debian-watch-could-verify-download.

debian/changelog

@@ -7,6 +7,7 @@ linux (4.18.6-2) UNRELEASED; urgency=medium
   * linux-headers: Stop linking the doc directory, which is not binNMU-safe
   * debian/copyright: Simplify exclusions to work with mk-origtargz
   * debian/{watch,bin/uscan-hook}: Drop uscan hook in favour of uupdate
+  * debian/watch: Add options for signature validation
 
   [ Salvatore Bonaccorso ]
   * mac80211: don't update the PM state of a peer upon a multicast frame

debian/watch

@@ -1,3 +1,4 @@
 # Find first non-rc tarball linked from kernel.org.
 version=3
+opts="pgpmode=mangle, pgpsigurlmangle=s|\.xz$|.sign|, decompress" \
 https://www.kernel.org/ .*/linux-([0-9.]+).tar.xz debian uupdate