This leads to double requiring "ext/psych/lib/psych/versions.rb" and
".ext/common/psych/versions.rb", which produces doxens of warnings
during `make test-all`.
20200608T123004Z.log.html.gz
```
[15092/20234]
TestRDocOptions#test_init_with_encoding/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:233: warning: already initialized constant Psych::LIBYAML_VERSION
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:233: warning: previous definition of LIBYAML_VERSION was here
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:235: warning: already initialized constant Psych::NOT_GIVEN
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:235: warning: previous definition of NOT_GIVEN was here
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:271: warning: method redefined; discarding old load
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:271: warning: previous definition of load was here
...
```
The same issue happened in fiddle.gemspec, and this change applies the
same fix as [fiddle](https://github.com/ruby/fiddle/issues/34).