Fix C++17 mode compilation with Apple clang

[traceon]

Fixes the following compilation error:

benchmark/test/output_test_helper.cc:206:28: error: no member named 'streampos' in 'std::__1::basic_ios<char>'

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[traceon]

I signed it!

[googlebot]

CLAs look good, thanks!

[EricWF]

LGTM, but I'll wait for the bots to cycle before merging this.

[coveralls]

Coverage remained the same at 89.288% when pulling 702c97b on traceon:fix-cpp17-compilation into b171791 on google:master.

[AppVeyorBot]

✅ Build benchmark 1567 completed (commit de980c4f19 by @traceon)

[dmah42]

thank you!