build: Threat unused-result warnings as errors

We don't have any in code now, so we should definitely ensure that we
won't introduce anymore unguarded calls.

meson.build

@@ -473,6 +473,7 @@ if cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Werror=init-self',
     '-Werror=missing-include-dirs',
     '-Werror=pointer-arith',
+    '-Werror=unused-result',
   ]
 
   warning_c_args = warning_common_args + [