Make runtime function return size_t

[mmcco]

It returns sizeof(dirent_t), so I'm not sure why its return type is int.
It's only used once, and that usage immediately casts it to usize.

[rust-highfive]

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[alexcrichton]

@bors: r+ cf03cee

[bors]

⌛ Testing commit cf03cee with merge 0047d08...

[bors]

💔 Test failed - auto-mac-64-nopt-t

[alexcrichton]

@bors: retry

On Sat, Dec 19, 2015 at 9:28 PM, bors notifications@github.com wrote:

[image: 💔] Test failed - auto-mac-64-nopt-t
http://buildbot.rust-lang.org/builders/auto-mac-64-nopt-t/builds/7495

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#30454 (comment).

[bors]

⌛ Testing commit cf03cee with merge d3aec9f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt