Fixes https://github.com/google/leveldb/issues/466

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189425354

gcc defaults to exporting all symbols, but other linkers do not. Adding
the LEVELDB_EXPORT macro allows a project to set LEVELDB_SHARED_LIBRARY
when building/linking with leveldb as a shared library.

This is to allow leveldb to be created as a shared library on all
platforms support by Chrome and enables a fix for
https://bugs.chromium.org/p/chromium/issues/detail?id=764810.

This also has the benefit of reducing the shared library size from
418863 to 380367 bytes (64-bit Linux).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171037148