linux/win: Fix wrong libexif target type in GN build.
linux/win: Fix wrong libexif target type in GN build.

ExifFunctions in image_metadata_extractor.cc loads libexif as
a dll/so, so the target needs to be a shared_library.

This isn't quite right on OS X yet, but it fixes the problem on other
platforms at least.

Review URL: https://codereview.chromium.org/1343453002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#348697}

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