meson: fail build when encountering unused variables

meson.build

@@ -405,6 +405,7 @@ possible_common_cc_flags = [
         '-Werror=shift-overflow=2',
         '-Werror=strict-flex-arrays',
         '-Werror=undef',
+        '-Werror=unused-variable',
         '-Wfloat-equal',
         # gperf prevents us from enabling this because it does not emit fallthrough
         # attribute with clang.