meson: Treat LLVM headers as a system dependency

This will suppress warnings from LLVM includes.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7632>

meson.build

@@ -1526,6 +1526,7 @@ if _llvm != 'disabled'
     static : not _shared_llvm,
     method : _llvm_method,
     fallback : ['llvm', 'dep_llvm'],
+    include_type : 'system',
   )
   with_llvm = dep_llvm.found()
 endif