meson: set a minimum Meson version

Without a version set meson will give no developer warnings, including
deprecations. 0.64 was selected as it's quite old, it's the newest
version supported by muon (a pure C Meson implementation), and there's
nothing complicated going on here.

Signed-off-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -2,6 +2,7 @@ project('nlohmann_json',
     'cpp',
     version : '3.12.0',
     license : 'MIT',
+    meson_version : '>= 0.64',
 )
 
 nlohmann_json_dep = declare_dependency(