Try to suppress warning on some CI

http://ci.rvm.jp/results/trunk-asserts-nopara@silicon-docker/1149270
```
/home/ko1/ruby/src/trunk-asserts-nopara/process.c: In function 'assert_close_on_exec':
/home/ko1/ruby/src/trunk-asserts-nopara/process.c:298:9: warning: ignoring return value of 'write', declared with attribute warn_unused_result [-Wunused-result]
         (void)write(2, m, sizeof(m) - 1);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

workaround from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425#c34

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64029 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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