Include sys/uio.h in check headers.

[peakji]

Without checking sys/uio.h, ac_cv_have_sys_uio_h will always be 0 @ snappy-stubs-public.h.in#L47

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[peakji]

I signed it!

[googlebot]

CLAs look good, thanks!

[kesla]

hey, any update on this? Want to see this merge so I can close kesla/node-snappy#145

[alkis]

This will be part of the next release.