meson: Build with more warnings by default

If we keep this module closer to warnings-clean, then it can be
submoduled into Meson projects that enable warnings by default without
having to override it to a lower warning-level.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -7,6 +7,7 @@ project(
   'c',
   default_options : [
     'c_std=gnu99',
+    'warning_level=3',
   ],
 )