Issue #6447: Meson 0.57 build fails.

No input to use. Also comparing to the autotools build, we were not
validating against the DTD, hence adding --valid now.
The other difference was that instead of looping through all XML menu
files, we are doing the check in one command. After a few tests, it
seems to work fine this way too and errors are still accurately given
(telling which file/line has issues), so I leave this difference as-is.

Also this rule is apparently never run (except when explicitly), which
is why we never discovered this bug until today. It would seem that
meson 0.57 would now run this rule, which is nice. But we should check
it is run appropriately (always when any dependencies are changed, never
when they are not), just like with the autotools build.

menus/meson.build

@@ -62,9 +62,9 @@ if xmllint.found()
   run_target('validate_menus',
     command: [
       xmllint,
-      '--noout',
+      '--noout', '--valid',
       '--path', meson.current_source_dir(),
-      '@INPUT@', menus_files,
+      menus_files,
     ],
   )
 endif