build: fix Meson warning about lix-clang-tidy not having meson_version

This fixes Meson's "Project does not target a minimum version but uses
feature introduced in '1.1': meson.options file" warning.

Silly Meson.

I also added a note in the top-level meson.build to indicate
`meson_version` is specified in more than one place.

Change-Id: I2c04278bb46a562a1c96cd2e5e4d9ce59ce8e125

subprojects/lix-clang-tidy/meson.build

@@ -1,4 +1,5 @@
 project('lix-clang-tidy', ['cpp', 'c'],
+  meson_version : '>=1.5.0',
   version : '0.1',
   default_options : ['warning_level=3', 'cpp_std=c++20']
 )