[calls] meson: Enable warning for unused variables

[Evangelos Ribeiro Tzaras <devrtz src gnome org>]

commit 5a15638db35c152e701699faac160347f25b0725
Author: Evangelos Ribeiro Tzaras <devrtz fortysixandtwo eu>
Date:   Mon May 9 11:08:14 2022 +0200

    meson: Enable warning for unused variables

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index a8de781c..31db00e3 100644
--- a/meson.build
+++ b/meson.build
@@ -103,6 +103,7 @@ test_c_args = [
   '-Wno-switch-enum',
   '-Wtype-limits',
   '-Wunused-function',
+  '-Wunused-variable',
   '-DGLIB_VERSION_MIN_REQUIRED=GLIB_VERSION_2_64',
   '-DGLIB_VERSION_MAX_ALLOWED=GLIB_VERSION_2_70',
   # see https://gitlab.gnome.org/GNOME/evolution-data-server/-/issues/332